Entity Framework Code First添加修改及删除外键关联实体

　　1、添加外键关联实体

　　1>、添加新的Province及City实体

using (var ctx = new PortalContext())
{
    var city1 = new City
    {
        CityNo = "10010",
        CityName = "测试城市1"
    };
    var city2 = new City
    {
        CityNo = "10020",
        CityName = "测试城市2"
    };

    var province = new Province
    {
        ProvinceNo = "10000",
        ProvinceName = "测试省份"
    };
    province.Cities.Add(city1);
    province.Cities.Add(city2);
    ctx.Provinces.Add(province);

    ctx.SaveChanges();
}

　　代码运行所执行的SQL语句：

exec sp_executesql N'insert [dbo].[Province]([ProvinceNo], [ProvinceName])
values (@0, @1)
select [ProvinceID]
from [dbo].[Province]
where @@ROWCOUNT > 0 and [ProvinceID] = scope_identity()',N'@0 nvarchar(10),@1 nvarchar(50)',@0=N'10000',@1=N'测试省份'

exec sp_executesql N'insert [dbo].[City]([ProvinceID], [CityNo], [CityName])
values (@0, @1, @2)
select [CityID]
from [dbo].[City]
where @@ROWCOUNT > 0 and [CityID] = scope_identity()',N'@0 int,@1 nvarchar(10),@2 nvarchar(50)',@0=39,@1=N'10010',@2=N'测试城市1'

exec sp_executesql N'insert [dbo].[City]([ProvinceID], [CityNo], [CityName])
values (@0, @1, @2)
select [CityID]
from [dbo].[City]
where @@ROWCOUNT > 0 and [CityID] = scope_identity()',N'@0 int,@1 nvarchar(10),@2 nvarchar(50)',@0=39,@1=N'10020',@2=N'测试城市2'

　　2>、添加新的Province实体及现有的City实体

using (var ctx = new PortalContext())
{
    var city1 = new City
    {
        CityNo = "10010",
        CityName = "测试城市1"
    };
    var city2 = new City
    {
        CityNo = "10020",
        CityName = "测试城市2"
    };
    var city3 = ctx.Cities.Find(36);

    var province = new Province
    {
        ProvinceNo = "10000",
        ProvinceName = "测试省份"
    };
    province.Cities.Add(city1);
    province.Cities.Add(city2);
    province.Cities.Add(city3);
    ctx.Provinces.Add(province);

    ctx.SaveChanges();
}

　　代码运行所执行的SQL语句：

exec sp_executesql N'SELECT 
[Limit1].[CityID] AS [CityID], 
[Limit1].[ProvinceID] AS [ProvinceID], 
[Limit1].[CityNo] AS [CityNo], 
[Limit1].[CityName] AS [CityName]
FROM ( SELECT TOP (2) 
    [Extent1].[CityID] AS [CityID], 
    [Extent1].[ProvinceID] AS [ProvinceID], 
    [Extent1].[CityNo] AS [CityNo], 
    [Extent1].[CityName] AS [CityName]
    FROM [dbo].[City] AS [Extent1]
    WHERE [Extent1].[CityID] = @p0
)  AS [Limit1]',N'@p0 int',@p0=36

exec sp_executesql N'insert [dbo].[Province]([ProvinceNo], [ProvinceName])
values (@0, @1)
select [ProvinceID]
from [dbo].[Province]
where @@ROWCOUNT > 0 and [ProvinceID] = scope_identity()',N'@0 nvarchar(10),@1 nvarchar(50)',@0=N'10000',@1=N'测试省份'

exec sp_executesql N'update [dbo].[City]
set [ProvinceID] = @0
where ([CityID] = @1)
',N'@0 int,@1 int',@0=40,@1=36

exec sp_executesql N'insert [dbo].[City]([ProvinceID], [CityNo], [CityName])
values (@0, @1, @2)
select [CityID]
from [dbo].[City]
where @@ROWCOUNT > 0 and [CityID] = scope_identity()',N'@0 int,@1 nvarchar(10),@2 nvarchar(50)',@0=40,@1=N'10010',@2=N'测试城市1'

exec sp_executesql N'insert [dbo].[City]([ProvinceID], [CityNo], [CityName])
values (@0, @1, @2)
select [CityID]
from [dbo].[City]
where @@ROWCOUNT > 0 and [CityID] = scope_identity()',N'@0 int,@1 nvarchar(10),@2 nvarchar(50)',@0=40,@1=N'10020',@2=N'测试城市2'

　　2、修改外键关联实体

　　1>、方式1

using (var ctx = new PortalContext())
{
    var city = ctx.Cities.Find(40);
    var province = ctx.Provinces.Find(10);
    city.Province = province;

    ctx.SaveChanges();
}

　　代码运行所执行的SQL语句：

exec sp_executesql N'SELECT 
[Limit1].[CityID] AS [CityID], 
[Limit1].[ProvinceID] AS [ProvinceID], 
[Limit1].[CityNo] AS [CityNo], 
[Limit1].[CityName] AS [CityName]
FROM ( SELECT TOP (2) 
    [Extent1].[CityID] AS [CityID], 
    [Extent1].[ProvinceID] AS [ProvinceID], 
    [Extent1].[CityNo] AS [CityNo], 
    [Extent1].[CityName] AS [CityName]
    FROM [dbo].[City] AS [Extent1]
    WHERE [Extent1].[CityID] = @p0
)  AS [Limit1]',N'@p0 int',@p0=40

exec sp_executesql N'SELECT 
[Limit1].[ProvinceID] AS [ProvinceID], 
[Limit1].[ProvinceNo] AS [ProvinceNo], 
[Limit1].[ProvinceName] AS [ProvinceName]
FROM ( SELECT TOP (2) 
    [Extent1].[ProvinceID] AS [ProvinceID], 
    [Extent1].[ProvinceNo] AS [ProvinceNo], 
    [Extent1].[ProvinceName] AS [ProvinceName]
    FROM [dbo].[Province] AS [Extent1]
    WHERE [Extent1].[ProvinceID] = @p0
)  AS [Limit1]',N'@p0 int',@p0=10

exec sp_executesql N'update [dbo].[City]
set [ProvinceID] = @0
where ([CityID] = @1)
',N'@0 int,@1 int',@0=10,@1=40

　　2>、方式2

using (var ctx = new PortalContext())
{
    var city = ctx.Cities.Find(41);
    var province = ctx.Provinces.Find(10);
    province.Cities.Add(city);

    ctx.SaveChanges();
}

　　方式2的实现方式与方式1的实现方式区别：在执行province.Cities.Add(city)时，会自动调用延迟加载，多执行一次从数据库中根据Province关联获取City的SQL语句。

exec sp_executesql N'SELECT 
[Extent1].[CityID] AS [CityID], 
[Extent1].[ProvinceID] AS [ProvinceID], 
[Extent1].[CityNo] AS [CityNo], 
[Extent1].[CityName] AS [CityName]
FROM [dbo].[City] AS [Extent1]
WHERE [Extent1].[ProvinceID] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=10

　　3>、方式3

using (var ctx = new PortalContext())
{
    var city = ctx.Cities.Find(42);
    var province = ctx.Provinces.Find(10);
    city.ProvinceID = province.ProvinceID;

    ctx.SaveChanges();
}

　　方式3与方式1在代码运行之后所执行的SQL语句是一样的。

　　3、删除外键关联实体

　　示例：对外键允许为空的外键表记录删除引用，在City表中外键ProvinceID引用Province表，并允许为空。

using (var ctx = new PortalContext())
{
    var city = ctx.Cities.Find(42);
    ctx.Entry(city)
        .Reference(c => c.Province)
        .Load();
    city.Province = null;

    ctx.SaveChanges();
}

　　代码运行所执行的SQL语句：

exec sp_executesql N'SELECT 
[Limit1].[CityID] AS [CityID], 
[Limit1].[ProvinceID] AS [ProvinceID], 
[Limit1].[CityNo] AS [CityNo], 
[Limit1].[CityName] AS [CityName]
FROM ( SELECT TOP (2) 
    [Extent1].[CityID] AS [CityID], 
    [Extent1].[ProvinceID] AS [ProvinceID], 
    [Extent1].[CityNo] AS [CityNo], 
    [Extent1].[CityName] AS [CityName]
    FROM [dbo].[City] AS [Extent1]
    WHERE [Extent1].[CityID] = @p0
)  AS [Limit1]',N'@p0 int',@p0=42

exec sp_executesql N'SELECT 
[Extent1].[ProvinceID] AS [ProvinceID], 
[Extent1].[ProvinceNo] AS [ProvinceNo], 
[Extent1].[ProvinceName] AS [ProvinceName]
FROM [dbo].[Province] AS [Extent1]
WHERE [Extent1].[ProvinceID] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=10

exec sp_executesql N'update [dbo].[City]
set [ProvinceID] = null
where ([CityID] = @0)
',N'@0 int',@0=42

　　根据外键删除与主键表的关联引用的另外一种实现方式：

using (var ctx = new PortalContext())
{
    var city = ctx.Cities.Find(42);
    ctx.Entry(city)
        .Reference(c => c.Province)
        .Load();
    city.ProvinceID = null;

    ctx.SaveChanges();
}