Django--Managers

Django--Managers

---

Manager

概念：

1.Manager是Django中的数据模型，可以通过manager进行对数据库的查询操作。可以看其结构它本身是一个空的类，其主要的功能来自于BaseManager，QuerySet。

2.在Django中的模型类中每个模型类中至少存在一个Manager。

3.默认情况下Django为每一个模型类添加了Manager其名称为objects。为了区别每个模型类你可以对每个模型类进行重新命名。方法如下，就是将对manage的类属性进行重新的赋值。

class Manager(BaseManager.from_queryset(QuerySet)):
    pass

class BaseManager

class BaseManager:
    # To retain order, track each time a Manager instance is created.
    creation_counter = 0

    # Set to True for the 'objects' managers that are automatically created.
    auto_created = False

    #: If set to True the manager will be serialized into migrations and will
    #: thus be available in e.g. RunPython operations.
    use_in_migrations = False

    def __new__(cls, *args, **kwargs):
        # Capture the arguments to make returning them trivial.
        obj = super().__new__(cls)
        obj._constructor_args = (args, kwargs)
        return obj

    def __init__(self):
        super().__init__()
        self._set_creation_counter()
        self.model = None
        self.name = None
        self._db = None
        self._hints = {}

    def __str__(self):
        """Return "app_label.model_label.manager_name"."""
        return '%s.%s' % (self.model._meta.label, self.name)

    def deconstruct(self):
        """
        Return a 5-tuple of the form (as_manager (True), manager_class,
        queryset_class, args, kwargs).

        Raise a ValueError if the manager is dynamically generated.
        """
        qs_class = self._queryset_class
        if getattr(self, '_built_with_as_manager', False):
            # using MyQuerySet.as_manager()
            return (
                True,  # as_manager
                None,  # manager_class
                '%s.%s' % (qs_class.__module__, qs_class.__name__),  # qs_class
                None,  # args
                None,  # kwargs
            )
        else:
            module_name = self.__module__
            name = self.__class__.__name__
            # Make sure it's actually there and not an inner class
            module = import_module(module_name)
            if not hasattr(module, name):
                raise ValueError(
                    "Could not find manager %s in %s.
"
                    "Please note that you need to inherit from managers you "
                    "dynamically generated with 'from_queryset()'."
                    % (name, module_name)
                )
            return (
                False,  # as_manager
                '%s.%s' % (module_name, name),  # manager_class
                None,  # qs_class
                self._constructor_args[0],  # args
                self._constructor_args[1],  # kwargs
            )

    def check(self, **kwargs):
        return []

    @classmethod
    def _get_queryset_methods(cls, queryset_class):
        def create_method(name, method):
            def manager_method(self, *args, **kwargs):
                return getattr(self.get_queryset(), name)(*args, **kwargs)
            manager_method.__name__ = method.__name__
            manager_method.__doc__ = method.__doc__
            return manager_method

        new_methods = {}
        for name, method in inspect.getmembers(queryset_class, predicate=inspect.isfunction):
            # Only copy missing methods.
            if hasattr(cls, name):
                continue
            # Only copy public methods or methods with the attribute `queryset_only=False`.
            queryset_only = getattr(method, 'queryset_only', None)
            if queryset_only or (queryset_only is None and name.startswith('_')):
                continue
            # Copy the method onto the manager.
            new_methods[name] = create_method(name, method)
        return new_methods

    @classmethod
    def from_queryset(cls, queryset_class, class_name=None):
        if class_name is None:
            class_name = '%sFrom%s' % (cls.__name__, queryset_class.__name__)
        class_dict = {
            '_queryset_class': queryset_class,
        }
        class_dict.update(cls._get_queryset_methods(queryset_class))
        return type(class_name, (cls,), class_dict)

    def contribute_to_class(self, model, name):
        if not self.name:
            self.name = name
        self.model = model

        setattr(model, name, ManagerDescriptor(self))

        model._meta.add_manager(self)

    def _set_creation_counter(self):
        """
        Set the creation counter value for this instance and increment the
        class-level copy.
        """
        self.creation_counter = BaseManager.creation_counter
        BaseManager.creation_counter += 1

    def db_manager(self, using=None, hints=None):
        obj = copy.copy(self)
        obj._db = using or self._db
        obj._hints = hints or self._hints
        return obj

    @property
    def db(self):
        return self._db or router.db_for_read(self.model, **self._hints)

    #######################
    # PROXIES TO QUERYSET #
    #######################

    def get_queryset(self):
        """
        Return a new QuerySet object. Subclasses can override this method to
        customize the behavior of the Manager.
        """
        return self._queryset_class(model=self.model, using=self._db, hints=self._hints)

    def all(self):
        # We can't proxy this method through the `QuerySet` like we do for the
        # rest of the `QuerySet` methods. This is because `QuerySet.all()`
        # works by creating a "copy" of the current queryset and in making said
        # copy, all the cached `prefetch_related` lookups are lost. See the
        # implementation of `RelatedManager.get_queryset()` for a better
        # understanding of how this comes into play.
        return self.get_queryset()

    def __eq__(self, other):
        return (
            isinstance(other, self.__class__) and
            self._constructor_args == other._constructor_args
        )

    def __hash__(self):
        return id(self)

class QuerySet

class QuerySet:
    """Represent a lazy database lookup for a set of objects."""

    def __init__(self, model=None, query=None, using=None, hints=None):
        self.model = model
        self._db = using
        self._hints = hints or {}
        self.query = query or sql.Query(self.model)
        self._result_cache = None
        self._sticky_filter = False
        self._for_write = False
        self._prefetch_related_lookups = ()
        self._prefetch_done = False
        self._known_related_objects = {}  # {rel_field: {pk: rel_obj}}
        self._iterable_class = ModelIterable
        self._fields = None

    def as_manager(cls):
        # Address the circular dependency between `Queryset` and `Manager`.
        from django.db.models.manager import Manager
        manager = Manager.from_queryset(cls)()
        manager._built_with_as_manager = True
        return manager
    as_manager.queryset_only = True
    as_manager = classmethod(as_manager)

    ########################
    # PYTHON MAGIC METHODS #
    ########################

    def __deepcopy__(self, memo):
        """Don't populate the QuerySet's cache."""
        obj = self.__class__()
        for k, v in self.__dict__.items():
            if k == '_result_cache':
                obj.__dict__[k] = None
            else:
                obj.__dict__[k] = copy.deepcopy(v, memo)
        return obj

    def __getstate__(self):
        # Force the cache to be fully populated.
        self._fetch_all()
        obj_dict = self.__dict__.copy()
        obj_dict[DJANGO_VERSION_PICKLE_KEY] = get_version()
        return obj_dict

    def __setstate__(self, state):
        msg = None
        pickled_version = state.get(DJANGO_VERSION_PICKLE_KEY)
        if pickled_version:
            current_version = get_version()
            if current_version != pickled_version:
                msg = (
                    "Pickled queryset instance's Django version %s does not "
                    "match the current version %s." % (pickled_version, current_version)
                )
        else:
            msg = "Pickled queryset instance's Django version is not specified."

        if msg:
            warnings.warn(msg, RuntimeWarning, stacklevel=2)

        self.__dict__.update(state)

    def __repr__(self):
        data = list(self[:REPR_OUTPUT_SIZE + 1])
        if len(data) > REPR_OUTPUT_SIZE:
            data[-1] = "...(remaining elements truncated)..."
        return '<%s %r>' % (self.__class__.__name__, data)

    def __len__(self):
        self._fetch_all()
        return len(self._result_cache)

    def __iter__(self):
        """
        The queryset iterator protocol uses three nested iterators in the
        default case:
            1. sql.compiler:execute_sql()
               - Returns 100 rows at time (constants.GET_ITERATOR_CHUNK_SIZE)
                 using cursor.fetchmany(). This part is responsible for
                 doing some column masking, and returning the rows in chunks.
            2. sql.compiler.results_iter()
               - Returns one row at time. At this point the rows are still just
                 tuples. In some cases the return values are converted to
                 Python values at this location.
            3. self.iterator()
               - Responsible for turning the rows into model objects.
        """
        self._fetch_all()
        return iter(self._result_cache)

    def __bool__(self):
        self._fetch_all()
        return bool(self._result_cache)

    def __getitem__(self, k):
        """Retrieve an item or slice from the set of results."""
        if not isinstance(k, (int, slice)):
            raise TypeError
        assert ((not isinstance(k, slice) and (k >= 0)) or
                (isinstance(k, slice) and (k.start is None or k.start >= 0) and
                 (k.stop is None or k.stop >= 0))), 
            "Negative indexing is not supported."

        if self._result_cache is not None:
            return self._result_cache[k]

        if isinstance(k, slice):
            qs = self._chain()
            if k.start is not None:
                start = int(k.start)
            else:
                start = None
            if k.stop is not None:
                stop = int(k.stop)
            else:
                stop = None
            qs.query.set_limits(start, stop)
            return list(qs)[::k.step] if k.step else qs

        qs = self._chain()
        qs.query.set_limits(k, k + 1)
        qs._fetch_all()
        return qs._result_cache[0]

    def __and__(self, other):
        self._merge_sanity_check(other)
        if isinstance(other, EmptyQuerySet):
            return other
        if isinstance(self, EmptyQuerySet):
            return self
        combined = self._chain()
        combined._merge_known_related_objects(other)
        combined.query.combine(other.query, sql.AND)
        return combined

    def __or__(self, other):
        self._merge_sanity_check(other)
        if isinstance(self, EmptyQuerySet):
            return other
        if isinstance(other, EmptyQuerySet):
            return self
        combined = self._chain()
        combined._merge_known_related_objects(other)
        combined.query.combine(other.query, sql.OR)
        return combined

    ####################################
    # METHODS THAT DO DATABASE QUERIES #
    ####################################

    def _iterator(self, use_chunked_fetch, chunk_size):
        yield from self._iterable_class(self, chunked_fetch=use_chunked_fetch, chunk_size=chunk_size)

    def iterator(self, chunk_size=2000):
        """
        An iterator over the results from applying this QuerySet to the
        database.
        """
        if chunk_size <= 0:
            raise ValueError('Chunk size must be strictly positive.')
        use_chunked_fetch = not connections[self.db].settings_dict.get('DISABLE_SERVER_SIDE_CURSORS')
        return self._iterator(use_chunked_fetch, chunk_size)

    def aggregate(self, *args, **kwargs):
        """
        Return a dictionary containing the calculations (aggregation)
        over the current queryset.

        If args is present the expression is passed as a kwarg using
        the Aggregate object's default alias.
        """
        if self.query.distinct_fields:
            raise NotImplementedError("aggregate() + distinct(fields) not implemented.")
        self._validate_values_are_expressions(args + tuple(kwargs.values()), method_name='aggregate')
        for arg in args:
            # The default_alias property raises TypeError if default_alias
            # can't be set automatically or AttributeError if it isn't an
            # attribute.
            try:
                arg.default_alias
            except (AttributeError, TypeError):
                raise TypeError("Complex aggregates require an alias")
            kwargs[arg.default_alias] = arg

        query = self.query.chain()
        for (alias, aggregate_expr) in kwargs.items():
            query.add_annotation(aggregate_expr, alias, is_summary=True)
            if not query.annotations[alias].contains_aggregate:
                raise TypeError("%s is not an aggregate expression" % alias)
        return query.get_aggregation(self.db, kwargs)

    def count(self):
        """
        Perform a SELECT COUNT() and return the number of records as an
        integer.

        If the QuerySet is already fully cached, return the length of the
        cached results set to avoid multiple SELECT COUNT(*) calls.
        """
        if self._result_cache is not None:
            return len(self._result_cache)

        return self.query.get_count(using=self.db)

    def get(self, *args, **kwargs):
        """
        Perform the query and return a single object matching the given
        keyword arguments.
        """
        clone = self.filter(*args, **kwargs)
        if self.query.can_filter() and not self.query.distinct_fields:
            clone = clone.order_by()
        num = len(clone)
        if num == 1:
            return clone._result_cache[0]
        if not num:
            raise self.model.DoesNotExist(
                "%s matching query does not exist." %
                self.model._meta.object_name
            )
        raise self.model.MultipleObjectsReturned(
            "get() returned more than one %s -- it returned %s!" %
            (self.model._meta.object_name, num)
        )

    def create(self, **kwargs):
        """
        Create a new object with the given kwargs, saving it to the database
        and returning the created object.
        """
        obj = self.model(**kwargs)
        self._for_write = True
        obj.save(force_insert=True, using=self.db)
        return obj

    def _populate_pk_values(self, objs):
        for obj in objs:
            if obj.pk is None:
                obj.pk = obj._meta.pk.get_pk_value_on_save(obj)

    def bulk_create(self, objs, batch_size=None):
        """
        Insert each of the instances into the database. Do *not* call
        save() on each of the instances, do not send any pre/post_save
        signals, and do not set the primary key attribute if it is an
        autoincrement field (except if features.can_return_ids_from_bulk_insert=True).
        Multi-table models are not supported.
        """
        # When you bulk insert you don't get the primary keys back (if it's an
        # autoincrement, except if can_return_ids_from_bulk_insert=True), so
        # you can't insert into the child tables which references this. There
        # are two workarounds:
        # 1) This could be implemented if you didn't have an autoincrement pk
        # 2) You could do it by doing O(n) normal inserts into the parent
        #    tables to get the primary keys back and then doing a single bulk
        #    insert into the childmost table.
        # We currently set the primary keys on the objects when using
        # PostgreSQL via the RETURNING ID clause. It should be possible for
        # Oracle as well, but the semantics for  extracting the primary keys is
        # trickier so it's not done yet.
        assert batch_size is None or batch_size > 0
        # Check that the parents share the same concrete model with the our
        # model to detect the inheritance pattern ConcreteGrandParent ->
        # MultiTableParent -> ProxyChild. Simply checking self.model._meta.proxy
        # would not identify that case as involving multiple tables.
        for parent in self.model._meta.get_parent_list():
            if parent._meta.concrete_model is not self.model._meta.concrete_model:
                raise ValueError("Can't bulk create a multi-table inherited model")
        if not objs:
            return objs
        self._for_write = True
        connection = connections[self.db]
        fields = self.model._meta.concrete_fields
        objs = list(objs)
        self._populate_pk_values(objs)
        with transaction.atomic(using=self.db, savepoint=False):
            objs_with_pk, objs_without_pk = partition(lambda o: o.pk is None, objs)
            if objs_with_pk:
                self._batched_insert(objs_with_pk, fields, batch_size)
            if objs_without_pk:
                fields = [f for f in fields if not isinstance(f, AutoField)]
                ids = self._batched_insert(objs_without_pk, fields, batch_size)
                if connection.features.can_return_ids_from_bulk_insert:
                    assert len(ids) == len(objs_without_pk)
                for obj_without_pk, pk in zip(objs_without_pk, ids):
                    obj_without_pk.pk = pk
                    obj_without_pk._state.adding = False
                    obj_without_pk._state.db = self.db

        return objs

    def get_or_create(self, defaults=None, **kwargs):
        """
        Look up an object with the given kwargs, creating one if necessary.
        Return a tuple of (object, created), where created is a boolean
        specifying whether an object was created.
        """
        lookup, params = self._extract_model_params(defaults, **kwargs)
        # The get() needs to be targeted at the write database in order
        # to avoid potential transaction consistency problems.
        self._for_write = True
        try:
            return self.get(**lookup), False
        except self.model.DoesNotExist:
            return self._create_object_from_params(lookup, params)

    def update_or_create(self, defaults=None, **kwargs):
        """
        Look up an object with the given kwargs, updating one with defaults
        if it exists, otherwise create a new one.
        Return a tuple (object, created), where created is a boolean
        specifying whether an object was created.
        """
        defaults = defaults or {}
        lookup, params = self._extract_model_params(defaults, **kwargs)
        self._for_write = True
        with transaction.atomic(using=self.db):
            try:
                obj = self.select_for_update().get(**lookup)
            except self.model.DoesNotExist:
                obj, created = self._create_object_from_params(lookup, params)
                if created:
                    return obj, created
            for k, v in defaults.items():
                setattr(obj, k, v() if callable(v) else v)
            obj.save(using=self.db)
        return obj, False

    def _create_object_from_params(self, lookup, params):
        """
        Try to create an object using passed params. Used by get_or_create()
        and update_or_create().
        """
        try:
            with transaction.atomic(using=self.db):
                params = {k: v() if callable(v) else v for k, v in params.items()}
                obj = self.create(**params)
            return obj, True
        except IntegrityError as e:
            try:
                return self.get(**lookup), False
            except self.model.DoesNotExist:
                pass
            raise e

    def _extract_model_params(self, defaults, **kwargs):
        """
        Prepare `lookup` (kwargs that are valid model attributes), `params`
        (for creating a model instance) based on given kwargs; for use by
        get_or_create() and update_or_create().
        """
        defaults = defaults or {}
        lookup = kwargs.copy()
        for f in self.model._meta.fields:
            if f.attname in lookup:
                lookup[f.name] = lookup.pop(f.attname)
        params = {k: v for k, v in kwargs.items() if LOOKUP_SEP not in k}
        params.update(defaults)
        property_names = self.model._meta._property_names
        invalid_params = []
        for param in params:
            try:
                self.model._meta.get_field(param)
            except exceptions.FieldDoesNotExist:
                # It's okay to use a model's property if it has a setter.
                if not (param in property_names and getattr(self.model, param).fset):
                    invalid_params.append(param)
        if invalid_params:
            raise exceptions.FieldError(
                "Invalid field name(s) for model %s: '%s'." % (
                    self.model._meta.object_name,
                    "', '".join(sorted(invalid_params)),
                ))
        return lookup, params

    def _earliest_or_latest(self, *fields, field_name=None):
        """
        Return the latest object, according to the model's
        'get_latest_by' option or optional given field_name.
        """
        if fields and field_name is not None:
            raise ValueError('Cannot use both positional arguments and the field_name keyword argument.')

        order_by = None
        if field_name is not None:
            warnings.warn(
                'The field_name keyword argument to earliest() and latest() '
                'is deprecated in favor of passing positional arguments.',
                RemovedInDjango30Warning,
            )
            order_by = (field_name,)
        elif fields:
            order_by = fields
        else:
            order_by = getattr(self.model._meta, 'get_latest_by')
            if order_by and not isinstance(order_by, (tuple, list)):
                order_by = (order_by,)
        if order_by is None:
            raise ValueError(
                "earliest() and latest() require either fields as positional "
                "arguments or 'get_latest_by' in the model's Meta."
            )

        assert self.query.can_filter(), 
            "Cannot change a query once a slice has been taken."
        obj = self._chain()
        obj.query.set_limits(high=1)
        obj.query.clear_ordering(force_empty=True)
        obj.query.add_ordering(*order_by)
        return obj.get()

    def earliest(self, *fields, field_name=None):
        return self._earliest_or_latest(*fields, field_name=field_name)

    def latest(self, *fields, field_name=None):
        return self.reverse()._earliest_or_latest(*fields, field_name=field_name)

    def first(self):
        """Return the first object of a query or None if no match is found."""
        for obj in (self if self.ordered else self.order_by('pk'))[:1]:
            return obj

    def last(self):
        """Return the last object of a query or None if no match is found."""
        for obj in (self.reverse() if self.ordered else self.order_by('-pk'))[:1]:
            return obj

    def in_bulk(self, id_list=None, *, field_name='pk'):
        """
        Return a dictionary mapping each of the given IDs to the object with
        that ID. If `id_list` isn't provided, evaluate the entire QuerySet.
        """
        assert self.query.can_filter(), 
            "Cannot use 'limit' or 'offset' with in_bulk"
        if field_name != 'pk' and not self.model._meta.get_field(field_name).unique:
            raise ValueError("in_bulk()'s field_name must be a unique field but %r isn't." % field_name)
        if id_list is not None:
            if not id_list:
                return {}
            filter_key = '{}__in'.format(field_name)
            batch_size = connections[self.db].features.max_query_params
            id_list = tuple(id_list)
            # If the database has a limit on the number of query parameters
            # (e.g. SQLite), retrieve objects in batches if necessary.
            if batch_size and batch_size < len(id_list):
                qs = ()
                for offset in range(0, len(id_list), batch_size):
                    batch = id_list[offset:offset + batch_size]
                    qs += tuple(self.filter(**{filter_key: batch}).order_by())
            else:
                qs = self.filter(**{filter_key: id_list}).order_by()
        else:
            qs = self._chain()
        return {getattr(obj, field_name): obj for obj in qs}

    def delete(self):
        """Delete the records in the current QuerySet."""
        assert self.query.can_filter(), 
            "Cannot use 'limit' or 'offset' with delete."

        if self._fields is not None:
            raise TypeError("Cannot call delete() after .values() or .values_list()")

        del_query = self._chain()

        # The delete is actually 2 queries - one to find related objects,
        # and one to delete. Make sure that the discovery of related
        # objects is performed on the same database as the deletion.
        del_query._for_write = True

        # Disable non-supported fields.
        del_query.query.select_for_update = False
        del_query.query.select_related = False
        del_query.query.clear_ordering(force_empty=True)

        collector = Collector(using=del_query.db)
        collector.collect(del_query)
        deleted, _rows_count = collector.delete()

        # Clear the result cache, in case this QuerySet gets reused.
        self._result_cache = None
        return deleted, _rows_count

    delete.alters_data = True
    delete.queryset_only = True

    def _raw_delete(self, using):
        """
        Delete objects found from the given queryset in single direct SQL
        query. No signals are sent and there is no protection for cascades.
        """
        return sql.DeleteQuery(self.model).delete_qs(self, using)
    _raw_delete.alters_data = True

    def update(self, **kwargs):
        """
        Update all elements in the current QuerySet, setting all the given
        fields to the appropriate values.
        """
        assert self.query.can_filter(), 
            "Cannot update a query once a slice has been taken."
        self._for_write = True
        query = self.query.chain(sql.UpdateQuery)
        query.add_update_values(kwargs)
        # Clear any annotations so that they won't be present in subqueries.
        query._annotations = None
        with transaction.atomic(using=self.db, savepoint=False):
            rows = query.get_compiler(self.db).execute_sql(CURSOR)
        self._result_cache = None
        return rows
    update.alters_data = True

    def _update(self, values):
        """
        A version of update() that accepts field objects instead of field names.
        Used primarily for model saving and not intended for use by general
        code (it requires too much poking around at model internals to be
        useful at that level).
        """
        assert self.query.can_filter(), 
            "Cannot update a query once a slice has been taken."
        query = self.query.chain(sql.UpdateQuery)
        query.add_update_fields(values)
        self._result_cache = None
        return query.get_compiler(self.db).execute_sql(CURSOR)
    _update.alters_data = True
    _update.queryset_only = False

    def exists(self):
        if self._result_cache is None:
            return self.query.has_results(using=self.db)
        return bool(self._result_cache)

    def _prefetch_related_objects(self):
        # This method can only be called once the result cache has been filled.
        prefetch_related_objects(self._result_cache, *self._prefetch_related_lookups)
        self._prefetch_done = True

    ##################################################
    # PUBLIC METHODS THAT RETURN A QUERYSET SUBCLASS #
    ##################################################

    def raw(self, raw_query, params=None, translations=None, using=None):
        if using is None:
            using = self.db
        return RawQuerySet(raw_query, model=self.model, params=params, translations=translations, using=using)

    def _values(self, *fields, **expressions):
        clone = self._chain()
        if expressions:
            clone = clone.annotate(**expressions)
        clone._fields = fields
        clone.query.set_values(fields)
        return clone

    def values(self, *fields, **expressions):
        fields += tuple(expressions)
        clone = self._values(*fields, **expressions)
        clone._iterable_class = ValuesIterable
        return clone

    def values_list(self, *fields, flat=False, named=False):
        if flat and named:
            raise TypeError("'flat' and 'named' can't be used together.")
        if flat and len(fields) > 1:
            raise TypeError("'flat' is not valid when values_list is called with more than one field.")

        field_names = {f for f in fields if not hasattr(f, 'resolve_expression')}
        _fields = []
        expressions = {}
        counter = 1
        for field in fields:
            if hasattr(field, 'resolve_expression'):
                field_id_prefix = getattr(field, 'default_alias', field.__class__.__name__.lower())
                while True:
                    field_id = field_id_prefix + str(counter)
                    counter += 1
                    if field_id not in field_names:
                        break
                expressions[field_id] = field
                _fields.append(field_id)
            else:
                _fields.append(field)

        clone = self._values(*_fields, **expressions)
        clone._iterable_class = (
            NamedValuesListIterable if named
            else FlatValuesListIterable if flat
            else ValuesListIterable
        )
        return clone

    def dates(self, field_name, kind, order='ASC'):
        """
        Return a list of date objects representing all available dates for
        the given field_name, scoped to 'kind'.
        """
        assert kind in ("year", "month", "day"), 
            "'kind' must be one of 'year', 'month' or 'day'."
        assert order in ('ASC', 'DESC'), 
            "'order' must be either 'ASC' or 'DESC'."
        return self.annotate(
            datefield=Trunc(field_name, kind, output_field=DateField()),
            plain_field=F(field_name)
        ).values_list(
            'datefield', flat=True
        ).distinct().filter(plain_field__isnull=False).order_by(('-' if order == 'DESC' else '') + 'datefield')

    def datetimes(self, field_name, kind, order='ASC', tzinfo=None):
        """
        Return a list of datetime objects representing all available
        datetimes for the given field_name, scoped to 'kind'.
        """
        assert kind in ("year", "month", "day", "hour", "minute", "second"), 
            "'kind' must be one of 'year', 'month', 'day', 'hour', 'minute' or 'second'."
        assert order in ('ASC', 'DESC'), 
            "'order' must be either 'ASC' or 'DESC'."
        if settings.USE_TZ:
            if tzinfo is None:
                tzinfo = timezone.get_current_timezone()
        else:
            tzinfo = None
        return self.annotate(
            datetimefield=Trunc(field_name, kind, output_field=DateTimeField(), tzinfo=tzinfo),
            plain_field=F(field_name)
        ).values_list(
            'datetimefield', flat=True
        ).distinct().filter(plain_field__isnull=False).order_by(('-' if order == 'DESC' else '') + 'datetimefield')

    def none(self):
        """Return an empty QuerySet."""
        clone = self._chain()
        clone.query.set_empty()
        return clone

    ##################################################################
    # PUBLIC METHODS THAT ALTER ATTRIBUTES AND RETURN A NEW QUERYSET #
    ##################################################################

    def all(self):
        """
        Return a new QuerySet that is a copy of the current one. This allows a
        QuerySet to proxy for a model manager in some cases.
        """
        return self._chain()

    def filter(self, *args, **kwargs):
        """
        Return a new QuerySet instance with the args ANDed to the existing
        set.
        """
        return self._filter_or_exclude(False, *args, **kwargs)

    def exclude(self, *args, **kwargs):
        """
        Return a new QuerySet instance with NOT (args) ANDed to the existing
        set.
        """
        return self._filter_or_exclude(True, *args, **kwargs)

    def _filter_or_exclude(self, negate, *args, **kwargs):
        if args or kwargs:
            assert self.query.can_filter(), 
                "Cannot filter a query once a slice has been taken."

        clone = self._chain()
        if negate:
            clone.query.add_q(~Q(*args, **kwargs))
        else:
            clone.query.add_q(Q(*args, **kwargs))
        return clone

    def complex_filter(self, filter_obj):
        """
        Return a new QuerySet instance with filter_obj added to the filters.

        filter_obj can be a Q object or a dictionary of keyword lookup
        arguments.

        This exists to support framework features such as 'limit_choices_to',
        and usually it will be more natural to use other methods.
        """
        if isinstance(filter_obj, Q):
            clone = self._chain()
            clone.query.add_q(filter_obj)
            return clone
        else:
            return self._filter_or_exclude(None, **filter_obj)

    def _combinator_query(self, combinator, *other_qs, all=False):
        # Clone the query to inherit the select list and everything
        clone = self._chain()
        # Clear limits and ordering so they can be reapplied
        clone.query.clear_ordering(True)
        clone.query.clear_limits()
        clone.query.combined_queries = (self.query,) + tuple(qs.query for qs in other_qs)
        clone.query.combinator = combinator
        clone.query.combinator_all = all
        return clone

    def union(self, *other_qs, all=False):
        # If the query is an EmptyQuerySet, combine all nonempty querysets.
        if isinstance(self, EmptyQuerySet):
            qs = [q for q in other_qs if not isinstance(q, EmptyQuerySet)]
            return qs[0]._combinator_query('union', *qs[1:], all=all) if qs else self
        return self._combinator_query('union', *other_qs, all=all)

    def intersection(self, *other_qs):
        # If any query is an EmptyQuerySet, return it.
        if isinstance(self, EmptyQuerySet):
            return self
        for other in other_qs:
            if isinstance(other, EmptyQuerySet):
                return other
        return self._combinator_query('intersection', *other_qs)

    def difference(self, *other_qs):
        # If the query is an EmptyQuerySet, return it.
        if isinstance(self, EmptyQuerySet):
            return self
        return self._combinator_query('difference', *other_qs)

    def select_for_update(self, nowait=False, skip_locked=False, of=()):
        """
        Return a new QuerySet instance that will select objects with a
        FOR UPDATE lock.
        """
        if nowait and skip_locked:
            raise ValueError('The nowait option cannot be used with skip_locked.')
        obj = self._chain()
        obj._for_write = True
        obj.query.select_for_update = True
        obj.query.select_for_update_nowait = nowait
        obj.query.select_for_update_skip_locked = skip_locked
        obj.query.select_for_update_of = of
        return obj

    def select_related(self, *fields):
        """
        Return a new QuerySet instance that will select related objects.

        If fields are specified, they must be ForeignKey fields and only those
        related objects are included in the selection.

        If select_related(None) is called, clear the list.
        """

        if self._fields is not None:
            raise TypeError("Cannot call select_related() after .values() or .values_list()")

        obj = self._chain()
        if fields == (None,):
            obj.query.select_related = False
        elif fields:
            obj.query.add_select_related(fields)
        else:
            obj.query.select_related = True
        return obj

    def prefetch_related(self, *lookups):
        """
        Return a new QuerySet instance that will prefetch the specified
        Many-To-One and Many-To-Many related objects when the QuerySet is
        evaluated.

        When prefetch_related() is called more than once, append to the list of
        prefetch lookups. If prefetch_related(None) is called, clear the list.
        """
        clone = self._chain()
        if lookups == (None,):
            clone._prefetch_related_lookups = ()
        else:
            for lookup in lookups:
                if isinstance(lookup, Prefetch):
                    lookup = lookup.prefetch_to
                lookup = lookup.split(LOOKUP_SEP, 1)[0]
                if lookup in self.query._filtered_relations:
                    raise ValueError('prefetch_related() is not supported with FilteredRelation.')
            clone._prefetch_related_lookups = clone._prefetch_related_lookups + lookups
        return clone

    def annotate(self, *args, **kwargs):
        """
        Return a query set in which the returned objects have been annotated
        with extra data or aggregations.
        """
        self._validate_values_are_expressions(args + tuple(kwargs.values()), method_name='annotate')
        annotations = OrderedDict()  # To preserve ordering of args
        for arg in args:
            # The default_alias property may raise a TypeError.
            try:
                if arg.default_alias in kwargs:
                    raise ValueError("The named annotation '%s' conflicts with the "
                                     "default name for another annotation."
                                     % arg.default_alias)
            except TypeError:
                raise TypeError("Complex annotations require an alias")
            annotations[arg.default_alias] = arg
        annotations.update(kwargs)

        clone = self._chain()
        names = self._fields
        if names is None:
            names = {f.name for f in self.model._meta.get_fields()}

        for alias, annotation in annotations.items():
            if alias in names:
                raise ValueError("The annotation '%s' conflicts with a field on "
                                 "the model." % alias)
            if isinstance(annotation, FilteredRelation):
                clone.query.add_filtered_relation(annotation, alias)
            else:
                clone.query.add_annotation(annotation, alias, is_summary=False)

        for alias, annotation in clone.query.annotations.items():
            if alias in annotations and annotation.contains_aggregate:
                if clone._fields is None:
                    clone.query.group_by = True
                else:
                    clone.query.set_group_by()
                break

        return clone

    def order_by(self, *field_names):
        """Return a new QuerySet instance with the ordering changed."""
        assert self.query.can_filter(), 
            "Cannot reorder a query once a slice has been taken."
        obj = self._chain()
        obj.query.clear_ordering(force_empty=False)
        obj.query.add_ordering(*field_names)
        return obj

    def distinct(self, *field_names):
        """
        Return a new QuerySet instance that will select only distinct results.
        """
        assert self.query.can_filter(), 
            "Cannot create distinct fields once a slice has been taken."
        obj = self._chain()
        obj.query.add_distinct_fields(*field_names)
        return obj

    def extra(self, select=None, where=None, params=None, tables=None,
              order_by=None, select_params=None):
        """Add extra SQL fragments to the query."""
        assert self.query.can_filter(), 
            "Cannot change a query once a slice has been taken"
        clone = self._chain()
        clone.query.add_extra(select, select_params, where, params, tables, order_by)
        return clone

    def reverse(self):
        """Reverse the ordering of the QuerySet."""
        if not self.query.can_filter():
            raise TypeError('Cannot reverse a query once a slice has been taken.')
        clone = self._chain()
        clone.query.standard_ordering = not clone.query.standard_ordering
        return clone

    def defer(self, *fields):
        """
        Defer the loading of data for certain fields until they are accessed.
        Add the set of deferred fields to any existing set of deferred fields.
        The only exception to this is if None is passed in as the only
        parameter, in which case removal all deferrals.
        """
        if self._fields is not None:
            raise TypeError("Cannot call defer() after .values() or .values_list()")
        clone = self._chain()
        if fields == (None,):
            clone.query.clear_deferred_loading()
        else:
            clone.query.add_deferred_loading(fields)
        return clone

    def only(self, *fields):
        """
        Essentially, the opposite of defer(). Only the fields passed into this
        method and that are not already specified as deferred are loaded
        immediately when the queryset is evaluated.
        """
        if self._fields is not None:
            raise TypeError("Cannot call only() after .values() or .values_list()")
        if fields == (None,):
            # Can only pass None to defer(), not only(), as the rest option.
            # That won't stop people trying to do this, so let's be explicit.
            raise TypeError("Cannot pass None as an argument to only().")
        for field in fields:
            field = field.split(LOOKUP_SEP, 1)[0]
            if field in self.query._filtered_relations:
                raise ValueError('only() is not supported with FilteredRelation.')
        clone = self._chain()
        clone.query.add_immediate_loading(fields)
        return clone

    def using(self, alias):
        """Select which database this QuerySet should execute against."""
        clone = self._chain()
        clone._db = alias
        return clone

    ###################################
    # PUBLIC INTROSPECTION ATTRIBUTES #
    ###################################

    @property
    def ordered(self):
        """
        Return True if the QuerySet is ordered -- i.e. has an order_by()
        clause or a default ordering on the model.
        """
        if self.query.extra_order_by or self.query.order_by:
            return True
        elif self.query.default_ordering and self.query.get_meta().ordering:
            return True
        else:
            return False

    @property
    def db(self):
        """Return the database used if this query is executed now."""
        if self._for_write:
            return self._db or router.db_for_write(self.model, **self._hints)
        return self._db or router.db_for_read(self.model, **self._hints)

    ###################
    # PRIVATE METHODS #
    ###################

    def _insert(self, objs, fields, return_id=False, raw=False, using=None):
        """
        Insert a new record for the given model. This provides an interface to
        the InsertQuery class and is how Model.save() is implemented.
        """
        self._for_write = True
        if using is None:
            using = self.db
        query = sql.InsertQuery(self.model)
        query.insert_values(fields, objs, raw=raw)
        return query.get_compiler(using=using).execute_sql(return_id)
    _insert.alters_data = True
    _insert.queryset_only = False

    def _batched_insert(self, objs, fields, batch_size):
        """
        A helper method for bulk_create() to insert the bulk one batch at a
        time. Insert recursively a batch from the front of the bulk and then
        _batched_insert() the remaining objects again.
        """
        if not objs:
            return
        ops = connections[self.db].ops
        batch_size = (batch_size or max(ops.bulk_batch_size(fields, objs), 1))
        inserted_ids = []
        for item in [objs[i:i + batch_size] for i in range(0, len(objs), batch_size)]:
            if connections[self.db].features.can_return_ids_from_bulk_insert:
                inserted_id = self._insert(item, fields=fields, using=self.db, return_id=True)
                if isinstance(inserted_id, list):
                    inserted_ids.extend(inserted_id)
                else:
                    inserted_ids.append(inserted_id)
            else:
                self._insert(item, fields=fields, using=self.db)
        return inserted_ids

    def _chain(self, **kwargs):
        """
        Return a copy of the current QuerySet that's ready for another
        operation.
        """
        obj = self._clone()
        if obj._sticky_filter:
            obj.query.filter_is_sticky = True
            obj._sticky_filter = False
        obj.__dict__.update(kwargs)
        return obj

    def _clone(self):
        """
        Return a copy of the current QuerySet. A lightweight alternative
        to deepcopy().
        """
        c = self.__class__(model=self.model, query=self.query.chain(), using=self._db, hints=self._hints)
        c._sticky_filter = self._sticky_filter
        c._for_write = self._for_write
        c._prefetch_related_lookups = self._prefetch_related_lookups[:]
        c._known_related_objects = self._known_related_objects
        c._iterable_class = self._iterable_class
        c._fields = self._fields
        return c

    def _fetch_all(self):
        if self._result_cache is None:
            self._result_cache = list(self._iterable_class(self))
        if self._prefetch_related_lookups and not self._prefetch_done:
            self._prefetch_related_objects()

    def _next_is_sticky(self):
        """
        Indicate that the next filter call and the one following that should
        be treated as a single filter. This is only important when it comes to
        determining when to reuse tables for many-to-many filters. Required so
        that we can filter naturally on the results of related managers.

        This doesn't return a clone of the current QuerySet (it returns
        "self"). The method is only used internally and should be immediately
        followed by a filter() that does create a clone.
        """
        self._sticky_filter = True
        return self

    def _merge_sanity_check(self, other):
        """Check that two QuerySet classes may be merged."""
        if self._fields is not None and (
                set(self.query.values_select) != set(other.query.values_select) or
                set(self.query.extra_select) != set(other.query.extra_select) or
                set(self.query.annotation_select) != set(other.query.annotation_select)):
            raise TypeError(
                "Merging '%s' classes must involve the same values in each case."
                % self.__class__.__name__
            )

    def _merge_known_related_objects(self, other):
        """
        Keep track of all known related objects from either QuerySet instance.
        """
        for field, objects in other._known_related_objects.items():
            self._known_related_objects.setdefault(field, {}).update(objects)

    def resolve_expression(self, *args, **kwargs):
        if self._fields and len(self._fields) > 1:
            # values() queryset can only be used as nested queries
            # if they are set up to select only a single field.
            raise TypeError('Cannot use multi-field values as a filter value.')
        query = self.query.resolve_expression(*args, **kwargs)
        query._db = self._db
        return query
    resolve_expression.queryset_only = True

    def _add_hints(self, **hints):
        """
        Update hinting information for use by routers. Add new key/values or
        overwrite existing key/values.
        """
        self._hints.update(hints)

    def _has_filters(self):
        """
        Check if this QuerySet has any filtering going on. This isn't
        equivalent with checking if all objects are present in results, for
        example, qs[1:]._has_filters() -> False.
        """
        return self.query.has_filters()

    @staticmethod
    def _validate_values_are_expressions(values, method_name):
        invalid_args = sorted(str(arg) for arg in values if not hasattr(arg, 'resolve_expression'))
        if invalid_args:
            raise TypeError(
                'QuerySet.%s() received non-expression(s): %s.' % (
                    method_name,
                    ', '.join(invalid_args),
                )
            )


class InstanceCheckMeta(type):
    def __instancecheck__(self, instance):
        return isinstance(instance, QuerySet) and instance.query.is_empty()


class EmptyQuerySet(metaclass=InstanceCheckMeta):
    """
    Marker class to checking if a queryset is empty by .none():
        isinstance(qs.none(), EmptyQuerySet) -> True
    """

    def __init__(self, *args, **kwargs):
        raise TypeError("EmptyQuerySet can't be instantiated")


class RawQuerySet:
    """
    Provide an iterator which converts the results of raw SQL queries into
    annotated model instances.
    """
    def __init__(self, raw_query, model=None, query=None, params=None,
                 translations=None, using=None, hints=None):
        self.raw_query = raw_query
        self.model = model
        self._db = using
        self._hints = hints or {}
        self.query = query or sql.RawQuery(sql=raw_query, using=self.db, params=params)
        self.params = params or ()
        self.translations = translations or {}

    def resolve_model_init_order(self):
        """Resolve the init field names and value positions."""
        converter = connections[self.db].introspection.column_name_converter
        model_init_fields = [f for f in self.model._meta.fields if converter(f.column) in self.columns]
        annotation_fields = [(column, pos) for pos, column in enumerate(self.columns)
                             if column not in self.model_fields]
        model_init_order = [self.columns.index(converter(f.column)) for f in model_init_fields]
        model_init_names = [f.attname for f in model_init_fields]
        return model_init_names, model_init_order, annotation_fields

    def __iter__(self):
        # Cache some things for performance reasons outside the loop.
        db = self.db
        compiler = connections[db].ops.compiler('SQLCompiler')(
            self.query, connections[db], db
        )

        query = iter(self.query)

        try:
            model_init_names, model_init_pos, annotation_fields = self.resolve_model_init_order()
            if self.model._meta.pk.attname not in model_init_names:
                raise InvalidQuery('Raw query must include the primary key')
            model_cls = self.model
            fields = [self.model_fields.get(c) for c in self.columns]
            converters = compiler.get_converters([
                f.get_col(f.model._meta.db_table) if f else None for f in fields
            ])
            if converters:
                query = compiler.apply_converters(query, converters)
            for values in query:
                # Associate fields to values
                model_init_values = [values[pos] for pos in model_init_pos]
                instance = model_cls.from_db(db, model_init_names, model_init_values)
                if annotation_fields:
                    for column, pos in annotation_fields:
                        setattr(instance, column, values[pos])
                yield instance
        finally:
            # Done iterating the Query. If it has its own cursor, close it.
            if hasattr(self.query, 'cursor') and self.query.cursor:
                self.query.cursor.close()

    def __repr__(self):
        return "<%s: %s>" % (self.__class__.__name__, self.query)

    def __getitem__(self, k):
        return list(self)[k]

    @property
    def db(self):
        """Return the database used if this query is executed now."""
        return self._db or router.db_for_read(self.model, **self._hints)

    def using(self, alias):
        """Select the database this RawQuerySet should execute against."""
        return RawQuerySet(
            self.raw_query, model=self.model,
            query=self.query.chain(using=alias),
            params=self.params, translations=self.translations,
            using=alias,
        )

    @cached_property
    def columns(self):
        """
        A list of model field names in the order they'll appear in the
        query results.
        """
        columns = self.query.get_columns()
        # Adjust any column names which don't match field names
        for (query_name, model_name) in self.translations.items():
            # Ignore translations for nonexistent column names
            try:
                index = columns.index(query_name)
            except ValueError:
                pass
            else:
                columns[index] = model_name
        return columns

    @cached_property
    def model_fields(self):
        """A dict mapping column names to model field names."""
        converter = connections[self.db].introspection.table_name_converter
        model_fields = {}
        for field in self.model._meta.fields:
            name, column = field.get_attname_column()
            model_fields[converter(column)] = field
        return model_fields


class Prefetch:
    def __init__(self, lookup, queryset=None, to_attr=None):
        # `prefetch_through` is the path we traverse to perform the prefetch.
        self.prefetch_through = lookup
        # `prefetch_to` is the path to the attribute that stores the result.
        self.prefetch_to = lookup
        if queryset is not None and not issubclass(queryset._iterable_class, ModelIterable):
            raise ValueError('Prefetch querysets cannot use values().')
        if to_attr:
            self.prefetch_to = LOOKUP_SEP.join(lookup.split(LOOKUP_SEP)[:-1] + [to_attr])

        self.queryset = queryset
        self.to_attr = to_attr

    def __getstate__(self):
        obj_dict = self.__dict__.copy()
        if self.queryset is not None:
            # Prevent the QuerySet from being evaluated
            obj_dict['queryset'] = self.queryset._chain(
                _result_cache=[],
                _prefetch_done=True,
            )
        return obj_dict

    def add_prefix(self, prefix):
        self.prefetch_through = prefix + LOOKUP_SEP + self.prefetch_through
        self.prefetch_to = prefix + LOOKUP_SEP + self.prefetch_to

    def get_current_prefetch_to(self, level):
        return LOOKUP_SEP.join(self.prefetch_to.split(LOOKUP_SEP)[:level + 1])

    def get_current_to_attr(self, level):
        parts = self.prefetch_to.split(LOOKUP_SEP)
        to_attr = parts[level]
        as_attr = self.to_attr and level == len(parts) - 1
        return to_attr, as_attr

    def get_current_queryset(self, level):
        if self.get_current_prefetch_to(level) == self.prefetch_to:
            return self.queryset
        return None

    def __eq__(self, other):
        if isinstance(other, Prefetch):
            return self.prefetch_to == other.prefetch_to
        return False

    def __hash__(self):
        return hash(self.__class__) ^ hash(self.prefetch_to)


def normalize_prefetch_lookups(lookups, prefix=None):
    """Normalize lookups into Prefetch objects."""
    ret = []
    for lookup in lookups:
        if not isinstance(lookup, Prefetch):
            lookup = Prefetch(lookup)
        if prefix:
            lookup.add_prefix(prefix)
        ret.append(lookup)
    return ret


def prefetch_related_objects(model_instances, *related_lookups):
    """
    Populate prefetched object caches for a list of model instances based on
    the lookups/Prefetch instances given.
    """
    if len(model_instances) == 0:
        return  # nothing to do

    # We need to be able to dynamically add to the list of prefetch_related
    # lookups that we look up (see below).  So we need some book keeping to
    # ensure we don't do duplicate work.
    done_queries = {}    # dictionary of things like 'foo__bar': [results]

    auto_lookups = set()  # we add to this as we go through.
    followed_descriptors = set()  # recursion protection

    all_lookups = normalize_prefetch_lookups(reversed(related_lookups))
    while all_lookups:
        lookup = all_lookups.pop()
        if lookup.prefetch_to in done_queries:
            if lookup.queryset:
                raise ValueError("'%s' lookup was already seen with a different queryset. "
                                 "You may need to adjust the ordering of your lookups." % lookup.prefetch_to)

            continue

        # Top level, the list of objects to decorate is the result cache
        # from the primary QuerySet. It won't be for deeper levels.
        obj_list = model_instances

        through_attrs = lookup.prefetch_through.split(LOOKUP_SEP)
        for level, through_attr in enumerate(through_attrs):
            # Prepare main instances
            if len(obj_list) == 0:
                break

            prefetch_to = lookup.get_current_prefetch_to(level)
            if prefetch_to in done_queries:
                # Skip any prefetching, and any object preparation
                obj_list = done_queries[prefetch_to]
                continue

            # Prepare objects:
            good_objects = True
            for obj in obj_list:
                # Since prefetching can re-use instances, it is possible to have
                # the same instance multiple times in obj_list, so obj might
                # already be prepared.
                if not hasattr(obj, '_prefetched_objects_cache'):
                    try:
                        obj._prefetched_objects_cache = {}
                    except (AttributeError, TypeError):
                        # Must be an immutable object from
                        # values_list(flat=True), for example (TypeError) or
                        # a QuerySet subclass that isn't returning Model
                        # instances (AttributeError), either in Django or a 3rd
                        # party. prefetch_related() doesn't make sense, so quit.
                        good_objects = False
                        break
            if not good_objects:
                break

            # Descend down tree

            # We assume that objects retrieved are homogeneous (which is the premise
            # of prefetch_related), so what applies to first object applies to all.
            first_obj = obj_list[0]
            to_attr = lookup.get_current_to_attr(level)[0]
            prefetcher, descriptor, attr_found, is_fetched = get_prefetcher(first_obj, through_attr, to_attr)

            if not attr_found:
                raise AttributeError("Cannot find '%s' on %s object, '%s' is an invalid "
                                     "parameter to prefetch_related()" %
                                     (through_attr, first_obj.__class__.__name__, lookup.prefetch_through))

            if level == len(through_attrs) - 1 and prefetcher is None:
                # Last one, this *must* resolve to something that supports
                # prefetching, otherwise there is no point adding it and the
                # developer asking for it has made a mistake.
                raise ValueError("'%s' does not resolve to an item that supports "
                                 "prefetching - this is an invalid parameter to "
                                 "prefetch_related()." % lookup.prefetch_through)

            if prefetcher is not None and not is_fetched:
                obj_list, additional_lookups = prefetch_one_level(obj_list, prefetcher, lookup, level)
                # We need to ensure we don't keep adding lookups from the
                # same relationships to stop infinite recursion. So, if we
                # are already on an automatically added lookup, don't add
                # the new lookups from relationships we've seen already.
                if not (lookup in auto_lookups and descriptor in followed_descriptors):
                    done_queries[prefetch_to] = obj_list
                    new_lookups = normalize_prefetch_lookups(reversed(additional_lookups), prefetch_to)
                    auto_lookups.update(new_lookups)
                    all_lookups.extend(new_lookups)
                followed_descriptors.add(descriptor)
            else:
                # Either a singly related object that has already been fetched
                # (e.g. via select_related), or hopefully some other property
                # that doesn't support prefetching but needs to be traversed.

                # We replace the current list of parent objects with the list
                # of related objects, filtering out empty or missing values so
                # that we can continue with nullable or reverse relations.
                new_obj_list = []
                for obj in obj_list:
                    if through_attr in getattr(obj, '_prefetched_objects_cache', ()):
                        # If related objects have been prefetched, use the
                        # cache rather than the object's through_attr.
                        new_obj = list(obj._prefetched_objects_cache.get(through_attr))
                    else:
                        try:
                            new_obj = getattr(obj, through_attr)
                        except exceptions.ObjectDoesNotExist:
                            continue
                    if new_obj is None:
                        continue
                    # We special-case `list` rather than something more generic
                    # like `Iterable` because we don't want to accidentally match
                    # user models that define __iter__.
                    if isinstance(new_obj, list):
                        new_obj_list.extend(new_obj)
                    else:
                        new_obj_list.append(new_obj)
                obj_list = new_obj_list


def get_prefetcher(instance, through_attr, to_attr):
    """
    For the attribute 'through_attr' on the given instance, find
    an object that has a get_prefetch_queryset().
    Return a 4 tuple containing:
    (the object with get_prefetch_queryset (or None),
     the descriptor object representing this relationship (or None),
     a boolean that is False if the attribute was not found at all,
     a boolean that is True if the attribute has already been fetched)
    """
    prefetcher = None
    is_fetched = False

    # For singly related objects, we have to avoid getting the attribute
    # from the object, as this will trigger the query. So we first try
    # on the class, in order to get the descriptor object.
    rel_obj_descriptor = getattr(instance.__class__, through_attr, None)
    if rel_obj_descriptor is None:
        attr_found = hasattr(instance, through_attr)
    else:
        attr_found = True
        if rel_obj_descriptor:
            # singly related object, descriptor object has the
            # get_prefetch_queryset() method.
            if hasattr(rel_obj_descriptor, 'get_prefetch_queryset'):
                prefetcher = rel_obj_descriptor
                if rel_obj_descriptor.is_cached(instance):
                    is_fetched = True
            else:
                # descriptor doesn't support prefetching, so we go ahead and get
                # the attribute on the instance rather than the class to
                # support many related managers
                rel_obj = getattr(instance, through_attr)
                if hasattr(rel_obj, 'get_prefetch_queryset'):
                    prefetcher = rel_obj
                if through_attr != to_attr:
                    # Special case cached_property instances because hasattr
                    # triggers attribute computation and assignment.
                    if isinstance(getattr(instance.__class__, to_attr, None), cached_property):
                        is_fetched = to_attr in instance.__dict__
                    else:
                        is_fetched = hasattr(instance, to_attr)
                else:
                    is_fetched = through_attr in instance._prefetched_objects_cache
    return prefetcher, rel_obj_descriptor, attr_found, is_fetched


def prefetch_one_level(instances, prefetcher, lookup, level):
    """
    Helper function for prefetch_related_objects().

    Run prefetches on all instances using the prefetcher object,
    assigning results to relevant caches in instance.

    Return the prefetched objects along with any additional prefetches that
    must be done due to prefetch_related lookups found from default managers.
    """
    # prefetcher must have a method get_prefetch_queryset() which takes a list
    # of instances, and returns a tuple:

    # (queryset of instances of self.model that are related to passed in instances,
    #  callable that gets value to be matched for returned instances,
    #  callable that gets value to be matched for passed in instances,
    #  boolean that is True for singly related objects,
    #  cache or field name to assign to,
    #  boolean that is True when the previous argument is a cache name vs a field name).

    # The 'values to be matched' must be hashable as they will be used
    # in a dictionary.

    rel_qs, rel_obj_attr, instance_attr, single, cache_name, is_descriptor = (
        prefetcher.get_prefetch_queryset(instances, lookup.get_current_queryset(level)))
    # We have to handle the possibility that the QuerySet we just got back
    # contains some prefetch_related lookups. We don't want to trigger the
    # prefetch_related functionality by evaluating the query. Rather, we need
    # to merge in the prefetch_related lookups.
    # Copy the lookups in case it is a Prefetch object which could be reused
    # later (happens in nested prefetch_related).
    additional_lookups = [
        copy.copy(additional_lookup) for additional_lookup
        in getattr(rel_qs, '_prefetch_related_lookups', ())
    ]
    if additional_lookups:
        # Don't need to clone because the manager should have given us a fresh
        # instance, so we access an internal instead of using public interface
        # for performance reasons.
        rel_qs._prefetch_related_lookups = ()

    all_related_objects = list(rel_qs)

    rel_obj_cache = {}
    for rel_obj in all_related_objects:
        rel_attr_val = rel_obj_attr(rel_obj)
        rel_obj_cache.setdefault(rel_attr_val, []).append(rel_obj)

    to_attr, as_attr = lookup.get_current_to_attr(level)
    # Make sure `to_attr` does not conflict with a field.
    if as_attr and instances:
        # We assume that objects retrieved are homogeneous (which is the premise
        # of prefetch_related), so what applies to first object applies to all.
        model = instances[0].__class__
        try:
            model._meta.get_field(to_attr)
        except exceptions.FieldDoesNotExist:
            pass
        else:
            msg = 'to_attr={} conflicts with a field on the {} model.'
            raise ValueError(msg.format(to_attr, model.__name__))

    # Whether or not we're prefetching the last part of the lookup.
    leaf = len(lookup.prefetch_through.split(LOOKUP_SEP)) - 1 == level

    for obj in instances:
        instance_attr_val = instance_attr(obj)
        vals = rel_obj_cache.get(instance_attr_val, [])

        if single:
            val = vals[0] if vals else None
            if as_attr:
                # A to_attr has been given for the prefetch.
                setattr(obj, to_attr, val)
            elif is_descriptor:
                # cache_name points to a field name in obj.
                # This field is a descriptor for a related object.
                setattr(obj, cache_name, val)
            else:
                # No to_attr has been given for this prefetch operation and the
                # cache_name does not point to a descriptor. Store the value of
                # the field in the object's field cache.
                obj._state.fields_cache[cache_name] = val
        else:
            if as_attr:
                setattr(obj, to_attr, vals)
            else:
                manager = getattr(obj, to_attr)
                if leaf and lookup.queryset is not None:
                    qs = manager._apply_rel_filters(lookup.queryset)
                else:
                    qs = manager.get_queryset()
                qs._result_cache = vals
                # We don't want the individual qs doing prefetch_related now,
                # since we have merged this into the current work.
                qs._prefetch_done = True
                obj._prefetched_objects_cache[cache_name] = qs
    return all_related_objects, additional_lookups

自定义管理器：

修改Manager的初始的QuerySet

在默认的情况下QuerySet返回的是整个模型类中的所有对象。例如 在shell下进行测试

>>> from queryset_demo.models import *
>>> Blog.objects.all()
<QuerySet [<Blog: change_new_name>, <Blog: create_test>, <Blog: Cheddar Talk>, <Blog: blog_3>, <Blog: Tom>, <Blog: new>, <Blog: new>]>
#想要获取所有书的书名
>>> for blg in Blog.objects.all():
...     print(blg.name)
... 
# 原谅所有的数据并没有太正规
change_new_name
create_test
Cheddar Talk
blog_3
Tom
new
new

通过自定义Manager管理器的方式试得返回的QuerySet为所有博客的名字

step-1在models.py文件中

class Blog_name(models.Manager):
    def get_queryset(self):
        return super().get_queryset().values_list('name',flat=True)



class Blog(models.Model):
    name = models.CharField(max_length=100)
    tagline = models.TextField()

    objects = models.Manager() # 默认的管理器
    get_blog_name = Blog_name() # 自定义管理器

    def __str__(self):
        return self.name

step-2在shell环境下测试

>>> from queryset_demo.models import *
>>> Blog.objects.all()
<QuerySet [<Blog: change_new_name>, <Blog: create_test>, <Blog: Cheddar Talk>, <Blog: blog_3>, <Blog: Tom>, <Blog: new>, <Blog: new>]>
>>> Blog.get_blog_name.all()
<QuerySet ['change_new_name', 'create_test', 'Cheddar Talk', 'blog_3', 'Tom', 'new', 'new']>

总结在 get_queryset(self)方法中返回的是一个查询集，所以我们在使用的时候可以在这个基础上使用QuerySet的Api返回我们想要的结果。