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(); 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 > );
var provinces = from p in ctx.Provinces.Include(p => p.Cities)
where p.ProvinceID >
select p;
var expr = from p in ctx.Provinces
where p.ProvinceID >
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();
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();
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();
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();
var cities = from c in province.Cities
where c.CityID >
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();
var expr = ctx.Entry(province)
.Collection(p => p.Cities)
.Query();
var cities = from c in expr
where c.CityID >
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();
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();
ctx.Entry(province)
.Collection(p => p.Cities)
.Query()
.Where(c => c.CityNo.Contains(""))
.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