EF5.x Code First 表关联与贪婪加载、延迟加载

晚上体验了一把EF的对象关联，确实强悍！

EF数据库对象的外键管理方式

1-指定导航属性，会自动生成外键，命名规则为：“表名_主键名”
2-默认情况下与导航属性的主键名称相同的字段会自动被标记为外键
3-通过[ForeignKey]标记指定实体类的属性为外键，
4-方式2的升级版，与导航属性的主键名称相同的字段会自动被标记为外键，然后指定字段对应的数据库中的列名

案例：

组织架构n : 1【组织用户关联】1 ：n用户

using System;
/**
 * 作者：陈杰
 * 时间：2012-08-10 00:25
 * 功能：公共字段接口
 **/
namespace ElegantWM.EntityModel
{
    public interface IEntity
    {
        Guid Id { get; set; }

        string CreateUser { get; set; }
        DateTime CreateTime { get; set; }
        string ModifyUser { get; set; }
        DateTime? ModifyTime { get; set; }
        Byte[] RowVersion { get; set; }
    }
}

using System;
using System.Collections;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
/**
 * 作者：陈杰
 * 时间：2012-08-10 00:25
 * 功能：公共字段接口实现类
 **/
namespace ElegantWM.EntityModel
{
    public class Entity:IEntity
    {
        public Guid Id { get; set; }

        public string CreateUser { get; set; }
        public DateTime CreateTime { get; set; }
        public string ModifyUser { get; set; }
        public DateTime? ModifyTime { get; set; }
        [Timestamp]
        public Byte[] RowVersion { get; set; }
    }
}

以上两个类为数据库实体的公共属性，但随着业务的复杂度增加，感觉提取实体的公共字段并不是很妥当，可适当折中考虑。

/*  组织架构实体  */
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ElegantWM.EntityModel
{
    public class WMS_Org : Entity
    {
        public WMS_Org() { }

        public string OrgName { get; set; }
        public string OrgDesc { get; set; }
        public int OrgOrder { get; set; }
        public string OrgFatherId { get; set; }
        public virtual ICollection<WMS_OrgUser> OrgUserIds { get; set; }
    }
}

/*  用户实体  */
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ElegantWM.EntityModel
{
    public class WMS_User : Entity
    {
        public WMS_User() { }

        public string UserName { get; set; }
        public string NickName { get; set; }
        public string UserPwd { get; set; }
        public string Sex { get; set; }
        public string Phone { get; set; }
        public string Email { get; set; }
        public string QQ { get; set; }
        public string Address { get; set; }
        public string Remark { get; set; }
        public virtual ICollection<WMS_OrgUser> UserOrgIds { get; set; }
    }
}

/* 用户和组织架构的关系 */
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Text;

namespace ElegantWM.EntityModel
{
    public class WMS_OrgUser : Entity
    {
        public WMS_OrgUser() { }
//3. 指定外键的方式
        public Guid OrgId { get; set; }
        [ForeignKey("OrgId")]
        public virtual WMS_Org Org { get; set; }

        public Guid UserId { get; set; }
        [ForeignKey("UserId")]
        public virtual WMS_User User { get; set; }

//1. 不指定外键，EF默认会生成 对象_表对象ID，也就是Org_Id，User_Id
        public virtual WMS_Org Org { get; set; }
        public virtual WMS_User User { get; set; }
    }

//2. 不适合本案例，因为所有对象的主键都是ID，自动识别
        public Guid Id { get; set; }
        public virtual WMS_Org Org { get; set; }
        ......
//4. 指定数据库真实外键名称
        [Column("OrgID")]
        public Guid Id { get; set; }
        public virtual WMS_Org Org { get; set; }
}
                

根据上面的4个原则，可以在组合出符合自己实际业务的关联类。

建立好了关联类后，需要注意的是，数据库中外键类型必须和主表的主键类型一致，当然外键关系你可以不用强制建立，但类型必须一直，否则会出错。

关联的对象注意声明为virtual，以供延迟加载。

当 EF 访问实体的子实体的时候是如何工作的呢？你的集合是 POCO 的集合，所以，在访问的时候没有事件发生，EF 通过从你定义的实体派生一个动态的对象，然后覆盖你的子实体集合访问属性来实现。这就是为什么需要标记你的子实体集合属性为 virtual 的原因。

执行EF的操作，验证

//1.创建关系
WMS_OrgUser ou = new WMS_OrgUser();
            ou.OrgId = Guid.Parse("79DDB55A-1587-4928-A312-58DA0C091459");
            ou.UserId = Guid.Parse("9E99FBCF-1517-4ACA-A81D-AE3535AF8E37");
            WMFactory.WMSOrgUser.Insert(ou);

//2. 根据关系，已知组织ID，获取用户
            WMS_OrgUser orguser = WMFactory.WMSOrgUser.GetById("7E632B62-B8EC-4D58-AE94-A412868146E7");
//正常获取了用户对象，正常打印用户昵称
            string name = orguser.User.NickName;
//3.测试贪婪加载，见文章后面
            orguser = WMFactory.WMSOrgUser.TestNoLazyLoading(Guid.Parse("7E632B62-B8EC-4D58-AE94-A412868146E7"));

//4.获取组织架构下的用户
            WMS_Org org = WMFactory.WMSOrg.GetById("79DDB55A-1587-4928-A312-58DA0C091459");
            ICollection<WMS_OrgUser> coll = org.OrgUserIds;
            orguser = coll.ToList()[0];
            string orguserid=orguser.Id.ToString();
            string orgusername = orguser.User.NickName;

关于延迟加载和贪婪加载

1.延迟加载，当对象使用的时候，再去数据库中加载，例如上面的orguser.User，当我调用这句话的时候，EF会到数据库中加载数据

EF默认是开启延迟加载的，禁用如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.ModelConfiguration.Conventions;
using ElegantWM.EntityModel;
using System.Configuration;
/**
 * 作者：陈杰
 * ＱＱ：710782046
 * 时间：2012-08-10
 * 功能：EF配置文件，支持多数据库
 **/
namespace ElegantWM.DAL
{
    public class DB : DbContext
    {
        //public static readonly log4net.ILog log = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

        //配置连接串,默认数据库DefaultDB
        public DB(string _ConnectStr)
            : base()
        {
            Database.Connection.ConnectionString = ConfigurationManager.ConnectionStrings[_ConnectStr].ToString();
            Database.SetInitializer<DB>(null);
             //这里哦
            this.Configuration.LazyLoadingEnabled = false;
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            //已经存在的数据库，不然会出现负数
            modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();            
            base.OnModelCreating(modelBuilder);
        }
        //注册到EF
        public DbSet<WMS_Role> wmRole { get; set; }
        public DbSet<WMS_User> wmUser { get; set; }
        public DbSet<WMS_Org> wmOrg { get; set; }
        
        public DbSet<WMS_Module> wmModule { get; set; }
        public DbSet<WMS_RoleModule> wmRoleModule { get; set; }
        public DbSet<WMS_UserRole> wmUserRole { get; set; }
        public DbSet<WMS_OrgUser> wmsOrgUser { get; set; }
    }
}

2.贪懒加载，一次性把数据组织好，加载到内存，类似join表

例如以上案例4，通过组织架构获取用户，EF延迟加载执行的sql如下：

exec sp_executesql N'SELECT TOP (2) 
[Extent1].[Id] AS [Id], 
[Extent1].[OrgName] AS [OrgName], 
[Extent1].[OrgDesc] AS [OrgDesc], 
[Extent1].[OrgOrder] AS [OrgOrder], 
[Extent1].[OrgFatherId] AS [OrgFatherId], 
[Extent1].[CreateUser] AS [CreateUser], 
[Extent1].[CreateTime] AS [CreateTime], 
[Extent1].[ModifyUser] AS [ModifyUser], 
[Extent1].[ModifyTime] AS [ModifyTime], 
[Extent1].[RowVersion] AS [RowVersion]
FROM [dbo].[WMS_Org] AS [Extent1]
WHERE [Extent1].[Id] = @p__linq__0',N'@p__linq__0 uniqueidentifier',@p__linq__0='79DDB55A-1587-4928-A312-58DA0C091459'



exec sp_executesql N'SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[OrgId] AS [OrgId], 
[Extent1].[UserId] AS [UserId], 
[Extent1].[CreateUser] AS [CreateUser], 
[Extent1].[CreateTime] AS [CreateTime], 
[Extent1].[ModifyUser] AS [ModifyUser], 
[Extent1].[ModifyTime] AS [ModifyTime], 
[Extent1].[RowVersion] AS [RowVersion]
FROM [dbo].[WMS_OrgUser] AS [Extent1]
WHERE [Extent1].[OrgId] = @EntityKeyValue1',N'@EntityKeyValue1 uniqueidentifier',@EntityKeyValue1='79DDB55A-1587-4928-A312-58DA0C091459'



exec sp_executesql N'SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[UserName] AS [UserName], 
[Extent1].[NickName] AS [NickName], 
[Extent1].[UserPwd] AS [UserPwd], 
[Extent1].[Sex] AS [Sex], 
[Extent1].[Phone] AS [Phone], 
[Extent1].[Email] AS [Email], 
[Extent1].[QQ] AS [QQ], 
[Extent1].[Address] AS [Address], 
[Extent1].[Remark] AS [Remark], 
[Extent1].[CreateUser] AS [CreateUser], 
[Extent1].[CreateTime] AS [CreateTime], 
[Extent1].[ModifyUser] AS [ModifyUser], 
[Extent1].[ModifyTime] AS [ModifyTime], 
[Extent1].[RowVersion] AS [RowVersion]
FROM [dbo].[WMS_User] AS [Extent1]
WHERE [Extent1].[Id] = @EntityKeyValue1',N'@EntityKeyValue1 uniqueidentifier',@EntityKeyValue1='9D001F8D-E304-4DB3-B5C0-7C5139E888A6'

贪婪加载的方式，如测试3，产生的SQL如下：

 public WMS_OrgUser TestNoLazyLoading(Guid id)
        {
            return DBSET.Include("User").Include("Org").FirstOrDefault(o => o.Id == id);
        }

exec sp_executesql N'SELECT 
1 AS [C1], 
[Limit1].[Id] AS [Id], 
[Limit1].[OrgId] AS [OrgId], 
[Limit1].[UserId] AS [UserId], 
[Limit1].[CreateUser] AS [CreateUser], 
[Limit1].[CreateTime] AS [CreateTime], 
[Limit1].[ModifyUser] AS [ModifyUser], 
[Limit1].[ModifyTime] AS [ModifyTime], 
[Limit1].[RowVersion] AS [RowVersion], 
[Extent2].[Id] AS [Id1], 
[Extent2].[UserName] AS [UserName], 
[Extent2].[NickName] AS [NickName], 
[Extent2].[UserPwd] AS [UserPwd], 
[Extent2].[Sex] AS [Sex], 
[Extent2].[Phone] AS [Phone], 
[Extent2].[Email] AS [Email], 
[Extent2].[QQ] AS [QQ], 
[Extent2].[Address] AS [Address], 
[Extent2].[Remark] AS [Remark], 
[Extent2].[CreateUser] AS [CreateUser1], 
[Extent2].[CreateTime] AS [CreateTime1], 
[Extent2].[ModifyUser] AS [ModifyUser1], 
[Extent2].[ModifyTime] AS [ModifyTime1], 
[Extent2].[RowVersion] AS [RowVersion1], 
[Extent3].[Id] AS [Id2], 
[Extent3].[OrgName] AS [OrgName], 
[Extent3].[OrgDesc] AS [OrgDesc], 
[Extent3].[OrgOrder] AS [OrgOrder], 
[Extent3].[OrgFatherId] AS [OrgFatherId], 
[Extent3].[CreateUser] AS [CreateUser2], 
[Extent3].[CreateTime] AS [CreateTime2], 
[Extent3].[ModifyUser] AS [ModifyUser2], 
[Extent3].[ModifyTime] AS [ModifyTime2], 
[Extent3].[RowVersion] AS [RowVersion2]
FROM    (SELECT TOP (1) [Extent1].[Id] AS [Id], [Extent1].[OrgId] AS [OrgId], [Extent1].[UserId] AS [UserId], [Extent1].[CreateUser] AS [CreateUser], [Extent1].[CreateTime] AS [CreateTime], [Extent1].[ModifyUser] AS [ModifyUser], [Extent1].[ModifyTime] AS [ModifyTime], [Extent1].[RowVersion] AS [RowVersion]
    FROM [dbo].[WMS_OrgUser] AS [Extent1]
    WHERE [Extent1].[Id] = @p__linq__0 ) AS [Limit1]
LEFT OUTER JOIN [dbo].[WMS_User] AS [Extent2] ON [Limit1].[UserId] = [Extent2].[Id]
LEFT OUTER JOIN [dbo].[WMS_Org] AS [Extent3] ON [Limit1].[OrgId] = [Extent3].[Id]',N'@p__linq__0 uniqueidentifier',@p__linq__0='7E632B62-B8EC-4D58-AE94-A412868146E7'

两种方式各有各的好处，可以根据具体情况使用