django orm 操作

django的orm使用方便，但对于一些复杂的操作，需要遵循特定的规范，特例特别记录一下：

模型：

from django.db import models

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

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Author(models.Model):
    name = models.CharField(max_length=50)
    email = models.EmailField()

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Entry(models.Model):
    blog = models.ForeignKey(Blog)
    headline = models.CharField(max_length=255)
    body_text = models.TextField()
    pub_date = models.DateField()
    mod_date = models.DateField()
    authors = models.ManyToManyField(Author)
    n_comments = models.IntegerField()
    n_pingbacks = models.IntegerField()
    rating = models.IntegerField()

    def __str__(self):              # __unicode__ on Python 2
        return self.headline

1、Entry关联Blog

    Entry.objects.all().select_related()

   对应sql：select entry.blog, ...  from entry inner join blog on entry.id=blog.id

2、Entry按关联模型中的字段进行排序

     Entry.objects.filter('blog__headline').all().select_related()
3、执行原始的sql

     Entry.objects.raw(sql) 
     参考：https://docs.djangoproject.com/en/1.10/topics/db/sql/
4、