（DevExpress2011 学习）XPO 学习之3 对象连接关系

7.       对象连接关系

对象之间有三种关联关系.它们分别是One-to-Many Relationships(一对多)

One-to-One Relationship(一对一) Many-to-many Relationship(多对多)

7.1     一对多

一对多是最常用的关联关系. 一个持久性对象A可以有多个关联的持久性对象B,但是持久性对象B仅有一个关联对象A.

 

public class TitleType  : XPObject  {
        string _typeName;
        [Size(50)]
        public string typeName
        {
            set
            {
                SetPropertyValue<string>("typeName", ref _typeName, value);
            }
            get
            {
                return _typeName;
             }
        }
     ……
        [Association(("TitleTypes"), typeof(Title))]
        public XPCollection Titles { get { return GetCollection("Titles"); } }
    ……
    }

public class Title : XPObject 
    {
        string _TitleName;
        [Size(50)]
        public string TitleName
        {
            set
            {
                SetPropertyValue<string>("TitleName", ref _TitleName, value);
            }
            get
            {
                return _TitleName;      
            }
        }

        string _TitleContent;
         public string TitleContent
        {
            set
            {
                SetPropertyValue<string>("TitleContent", ref _TitleContent, value);
            }
            get
            {
                return _TitleContent;    
            }
        }
        ….
        [Association("TitleTypes")]
        public TitleType  titleType;

……….
}

在这个例子,Association属性(AssociationAttribute) 应用于 TitleType .Titles 和Title.TitleTypes

 

在这里,Association 属性应用于TitleType .Titles 和Title.TitleTypes 属性去定义两者之间的联系. 当一个对象与另一个对象关联, 多的一方关联必须定义一个XPCollection 或者XPCollection<T > 类.

 

在使用中, 如果类只有一个连接,你可以省去 [Association] 属性, 例如:

[Association("TitleTypes")]

        public TitleType titleType;

但是如果该类出了和Title类连接,它还与其他类关联,你必须指定该连接属性名取协助XPO解决关联.

 

7.2     一对一

 当2个对象参与关联,它有一个属性去指向他们持久性对象. 它必须需要对Setter 方法编写额外的代码来确信他们关系的完整性. 下例中直接引用帮助中的例子

// Represents the Building class which refers to the building's owner.
public class Building : XPObject {
    Person owner = null;
    public Person Owner {
        get { return owner; }
        set {
            if(owner == value)
                return;

            // Store a reference to the former owner.
            Person prevOwner = owner;
            owner = value;

            if(IsLoading) return;

            // Remove an owner's reference to this building, if exists.
            if(prevOwner != null && prevOwner.House == this)
                prevOwner.House = null;

            // Specify that the building is a new owner's house.
            if(owner != null)
                owner.House = this;
            OnChanged("Owner");
        }
    }
}

// Represents the Person class which refers to the person's house.
public class Person : XPObject {
    Building house = null;
    public Building House {
        get { return house; }
        set {
            if(house == value)
                return;

            // Store a reference to the person's former house.
            Building prevHouse = house;
            house = value;

            if(IsLoading) return;

            // Remove a reference to the house's owner, if the person is its owner.
            if(prevHouse != null && prevHouse.Owner == this)
                prevHouse.Owner = null;

            // Specify the person as a new owner of the house.
            if(house != null)
                house.Owner = this;

            OnChanged("House");
        }
    }
}

 

7.3     多对多

class History : XPObject 
    {
        string _historyName;
        [Size(50)]
        public string historyName
        {
            set
            {
                SetPropertyValue<string>("historyName", ref _historyName, value);
            }
            get
            {
                return _historyName;
            }
        }
        [Association(("HistoryTitles"),typeof (Title))]
        public XPCollection Titles { get { return GetCollection("Titles"); } }
          public History() : base() {
            // This constructor is used when an object is loaded from a persistent storage.
            // Do not place any code here.
        }
        ……
   
    }

public class Title : XPObject 
    {
        string _TitleName;
        [Size(50)]
        public string TitleName
        {
            set
            {
                SetPropertyValue<string>("TitleName", ref _TitleName, value);
            }
            get
            {
                return _TitleName;      
            }
        }

        string _TitleContent;
         public string TitleContent
        {
            set
            {
                SetPropertyValue<string>("TitleContent", ref _TitleContent, value);
            }
            get
            {
                return _TitleContent;    
            }
        }
        
       
      [Association("TitleTypes")]
        public TitleType  titleType;

      [Association(("HistoryTitles"), typeof(History))]
      public XPCollection Historys { get { return GetCollection("Historys"); } }

        public Title() : base() {
            // This constructor is used when an object is loaded from a persistent storage.
            // Do not place any code here.
        }
…….
    }

 

实际上, XPO会在你的数据库中除了生成想对应的这2个类以外,它还会额外的增加他们两者之间的一个关联类, 该实例中生成的数据表如下图: