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