Entity Framework Code First实体关联数据加载

　　在项目过程中，两个实体数据之间在往往并非完全独立的，而是存在一定的关联关系，如一对一、一对多及多对多等关联。存在关联关系的实体，经常根据一个实体的实例来查询获取与之关联的另外实体的实例。

　　Entity Framework常用处理数据关联加载的方式有3种：延迟加载（Lazy Loading）、贪婪加载（Eager Loading）以及显示加载（Explicit Loading）。

　　1、延迟加载（Lazy Loading）

　　延迟加载是项目应用中常见的方式，Entity Framework在需要时可以自动为一个实体的实例获取关联的数据。

　　Entity Framework自动延迟加载需要满足的条件：

　　1>、POCO类必须是public而非sealed；

　　2>、集合属性必须的Virtual修饰的，这样Entity Framework才能Override以包含延迟加载的逻辑。

　　示例：

　　文件类Province.cs：

using System;
using System.Collections.Generic;

namespace Portal.Models
{
    public class Province
    {
        public Province()
        {
            this.Cities = new List<City>();
        }

        public int ProvinceID { get; set; }
        public string ProvinceNo { get; set; }
        public string ProvinceName { get; set; }
        public virtual ICollection<City> Cities { get; set; }
    }
}

　　文件类City.cs：

using System;
using System.Collections.Generic;

namespace Portal.Models
{
    public class City
    {
        public int CityID { get; set; }
        public Nullable<int> ProvinceID { get; set; }
        public string CityNo { get; set; }
        public string CityName { get; set; }
        public virtual Province Province { get; set; }
    }
}

　　文件类Program.cs：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using System.Data.Entity;

using Portal.Models;

namespace Portal
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var ctx = new PortalContext())
            {
                var province = ctx.Provinces.Find(3);

                foreach (var city in province.Cities)
                {
                    Console.WriteLine(city.CityName);
                }
            }
        }
    }
}

　　以上代码在运行之后，执行了两条SQL语句，分别用于读取单条Province记录及与该条记录相关联的City记录。

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=3

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=3

　　延迟加载的不足：

　　延迟加载使用简单，应用程序不需要真正知道数据已经被从数据库中加载出来，但只要将可能导致大量的SQL查询被发送到数据库中执行，数据库进行了不必要的查询。

　　2、贪婪加载（Eager Loading）

　　贪婪加载：使用Include加载关联的数据，在Entity Framework进行查询时，即同时加载出关联的数据。Entity Framework贪婪加载将使用一条JOIN的SQL语句进行查询。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using System.Data.Entity;

using Portal.Models;

namespace Portal
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var ctx = new PortalContext())
            {
                var provinces = ctx.Provinces
                    .Include(p => p.Cities);

                foreach (var province in provinces)
                {
                    foreach (var city in province.Cities)
                    {
                        Console.WriteLine("{0}-{1}", province.ProvinceName, city.CityName);
                    }
                }
            }
        }
    }
}

　　运行代码所执行的SQL语句：

SELECT 
[Project1].[ProvinceID] AS [ProvinceID], 
[Project1].[ProvinceNo] AS [ProvinceNo], 
[Project1].[ProvinceName] AS [ProvinceName], 
[Project1].[C1] AS [C1], 
[Project1].[CityID] AS [CityID], 
[Project1].[ProvinceID1] AS [ProvinceID1], 
[Project1].[CityNo] AS [CityNo], 
[Project1].[CityName] AS [CityName]
FROM ( SELECT 
    [Extent1].[ProvinceID] AS [ProvinceID], 
    [Extent1].[ProvinceNo] AS [ProvinceNo], 
    [Extent1].[ProvinceName] AS [ProvinceName], 
    [Extent2].[CityID] AS [CityID], 
    [Extent2].[ProvinceID] AS [ProvinceID1], 
    [Extent2].[CityNo] AS [CityNo], 
    [Extent2].[CityName] AS [CityName], 
    CASE WHEN ([Extent2].[CityID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
    FROM  [dbo].[Province] AS [Extent1]
    LEFT OUTER JOIN [dbo].[City] AS [Extent2] ON [Extent1].[ProvinceID] = [Extent2].[ProvinceID]
)  AS [Project1]
ORDER BY [Project1].[ProvinceID] ASC, [Project1].[C1] ASC

　　Include语句可以在一次查询中使用多次。

ctx.Categories
    .Include(c => c.Products)
    .Include(c => c.News);

　　贪婪加载的不足：

　　贪婪加载的优势在于仅执行1次SQL查询即返回所需要的结果。但使用JOIN查询在数据库记录条数较多时，多条简单的SQL查询往往比一条复杂的JOIN查询效率要好。

　　使用Include的LINQ查询

var provinces = ctx.Provinces
    .Include(p => p.Cities)
    .Where(p => p.ProvinceID > 10);

var provinces = from p in ctx.Provinces.Include(p => p.Cities)
                where p.ProvinceID > 10
                select p;

var expr = from p in ctx.Provinces
           where p.ProvinceID > 10
           select p;
var provinces = expr.Include(p => p.Cities);

　　3、显示加载（Explicit Loading）

　　显示加载与延迟加载一样，采用主数据与关联数据独立分开加载。显示加载与延迟加载的区别在于显示加载不会自动的加载关联数据，需要调用方法去加载。

　　显示加载是使用DbContext.Entry方法来实现的，Entry方法可以获取DbContext中的实体信息。在使用Entry获取实体信息之后，可以使用Collection或Reference方法获取和操作实体关联的集合属性。如使用Load方法查询集合属性。

　　示例1：显示加载，使用Collection获取集合属性

using (var ctx = new PortalContext())
{
    var province = ctx.Provinces.Find(3);
    ctx.Entry(province)
        .Collection(p => p.Cities)
        .Query()
        .Load();

    foreach (var city in province.Cities)
    {
        Console.WriteLine("{0}-{1}", province.ProvinceName, city.CityName);
    }
}

　　上面的代码运行之后，执行的SQL语句：

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=3

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=3

　　从代码运行所执行的SQL语句可以看出，其查询数据库的方式与延迟加载是相同的。

　　示例2：显示加载，使用Reference方法获取引用属性

using (var ctx = new PortalContext())
{
    var city = ctx.Cities.Find(10);
    ctx.Entry(city).Reference(c => c.Province);
    Console.WriteLine("{0}-{1}", city.Province.ProvinceName, city.CityName);
}

　　上面的代码运行之后执行的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=10

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=3

　　检查集合属性是否已经加载：

using (var ctx = new PortalContext())
{
    var province = ctx.Provinces.Find(3);
    Console.WriteLine("Before load：{0}", ctx.Entry(province).Collection(p => p.Cities).IsLoaded);

    ctx.Entry(province)
        .Collection(p => p.Cities)
        .Load();

    Console.WriteLine("After load：{0}", ctx.Entry(province).Collection(p => p.Cities).IsLoaded);
}

　　4、集合属性查询

　　在使用Entry和Collection方法获取到实体集合属性之后，可以使用Query方法对集合属性进行查询。

　　示例：从内存中查询集合属性

using (var ctx = new PortalContext())
{
    var province = ctx.Provinces.Find(5);
    var cities = from c in province.Cities
                 where c.CityID > 30
                 select c;
    foreach (var city in cities)
    {
        Console.WriteLine("{0}-{1}", city.CityID, city.CityName);
    }
}

　　代码运行之后执行的SQL语句：

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=5

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=5

　　从对City表执行的SQL语句可以看出，其并对加入查询条件，仅只是对之前通过延长加载方式将Province实体的Cities集合属性载人到内存中，然后通过对内存中的Cities数据进行内存查询，并未生成新的包含查询条件的SQL语句。

　　示例：在数据库中查询集合属性

using (var ctx = new PortalContext())
{
    var province = ctx.Provinces.Find(5);
    var expr = ctx.Entry(province)
        .Collection(p => p.Cities)
        .Query();
    var cities = from c in expr
                 where c.CityID > 30
                 select c;
    foreach (var city in cities)
    {
        Console.WriteLine("{0}-{1}", city.CityID, city.CityName);
    }
}

　　代码运行之后执行的SQL语句：

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=5

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) AND ([Extent1].[CityID] > 30)',N'@EntityKeyValue1 int',@EntityKeyValue1=5

　　集合属性Count查询

using (var ctx = new PortalContext())
{
    var province = ctx.Provinces.Find(5);
    var expr = ctx.Entry(province)
        .Collection(p => p.Cities)
        .Query();
    Console.WriteLine(expr.Count());
}

　　代码运行生成的SQL语句：

exec sp_executesql N'SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
    COUNT(1) AS [A1]
    FROM [dbo].[City] AS [Extent1]
    WHERE [Extent1].[ProvinceID] = @EntityKeyValue1
)  AS [GroupBy1]',N'@EntityKeyValue1 int',@EntityKeyValue1=5

　　显示加载集合属性的子集：

using (var ctx = new PortalContext())
{
    var province = ctx.Provinces.Find(5);
    ctx.Entry(province)
        .Collection(p => p.Cities)
        .Query()
        .Where(c => c.CityNo.Contains("3"))
        .Load();
}

　　代码运行后生成的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) AND ([Extent1].[CityNo] LIKE N''%3%'')',N'@EntityKeyValue1 int',@EntityKeyValue1=5