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[ASP.net教程]EF里单个实体的增查改删以及主从表关联数据的各种增删 改查


本文目录


  • EF对单个实体的增查改删
    • 增加单个实体
    • 查询单个实体
    • 修改单个实体
    • 删除单个实体
  • EF里主从表关联数据的各种增删改查
    • 增加(增加从表数据、增加主从表数据)
    • 查询(根据主表找从表数据、根据从表找主表数据)
    • 修改(修改从表的外键)
    • 删除(删除主从表关系、删除主表数据、删除主从表数据、修改从表数据外键)
  • 补充内容
    • SaveChanges方法提交多次操作
    • DbSet.Add方法返回当前实体
  • 源码和系列文章导航


注:本章节多次演示了各种删除,要重复查看效果,需要解开注释初始化数据方法。

 

一、EF对单个实体的增查改删

 

 增加单个实体:

    /// <summary>    /// 增加单个实体    /// </summary>    private static void AddMachuPicchu()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var mauchuPicchu = new DbContexts.Model.Destination        {          Name = "Machu Picchu",          Country = "Peru"        };        context.Destinations.Add(mauchuPicchu);        context.SaveChanges();      }    }

产生的insert sql:

exec sp_executesql N'insert [baga].[Locations]([LocationName], [Country], [Description], [Photo], [TravelWarnings], [ClimateInfo])values (@0, @1, null, null, null, null)select [LocationID]from [baga].[Locations]where @@ROWCOUNT > 0 and [LocationID] = scope_identity()',N'@0 nvarchar(200),@1 nvarchar(max) ',@0=N'Machu Picchu',@1=N'Peru'

 

查询单个实体:

    /// <summary>    /// 查询单个实体    /// </summary>    private static void GetGreatBarrierReef()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var destination = context.Destinations.Find(4);        Console.WriteLine(destination.Name);      }    }

生成的select sql(find方法生成的查询sql略复杂点,普通的linq查询或者Lambda表达式写法就简单许多了):

exec sp_executesql N'SELECT [Limit1].[LocationID] AS [LocationID], [Limit1].[LocationName] AS [LocationName], [Limit1].[Country] AS [Country], [Limit1].[Description] AS [Description], [Limit1].[Photo] AS [Photo], [Limit1].[TravelWarnings] AS [TravelWarnings], [Limit1].[ClimateInfo] AS [ClimateInfo]FROM ( SELECT TOP (2)   [Extent1].[LocationID] AS [LocationID],   [Extent1].[LocationName] AS [LocationName],   [Extent1].[Country] AS [Country],   [Extent1].[Description] AS [Description],   [Extent1].[Photo] AS [Photo],   [Extent1].[TravelWarnings] AS [TravelWarnings],   [Extent1].[ClimateInfo] AS [ClimateInfo]  FROM [baga].[Locations] AS [Extent1]  WHERE [Extent1].[LocationID] = @p0) AS [Limit1]',N'@p0 int',@p0=4

 

修改单个实体:

    /// <summary>    /// 修改单个实体    /// </summary>    private static void ChangeGrandCanyon()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var canyon = (from d in context.Destinations               where d.Name == "Grand Canyon"               select d).Single();        canyon.Description = "227 mile long canyon.";        context.SaveChanges();      }    }

产生的update sql:

exec sp_executesql N'update [baga].[Locations]set [Description] = @0where ([LocationID] = @1)',N'@0 nvarchar(500),@1 int',@0=N'227 mile long canyon.',@1=1

 

删除单个实体:

    /// <summary>    /// 删除单个实体    /// </summary>    private static void DeleteWineGlassBay()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var bay = (from d in context.Destinations              where d.Name == "Wine Glass Bay"              select d).Single();        context.Destinations.Remove(bay);        context.SaveChanges();      }    }

产生的delete sql:

exec sp_executesql N'delete [baga].[Locations]where ([LocationID] = @0)',N'@0 int',@0=3

删除方法这样写可能有点效率问题:要删除一个实体,只要知道它的id就可以了,但是上面的方法却先加载了这个实体到内存中,这个是多余的步骤。我们使用attach方法:

    /// <summary>    /// 删除单个实体attach    /// </summary>    private static void DeleteWineGlassBayAttach()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var toDelete = new DbContexts.Model.Destination { DestinationId = 2 };        context.Destinations.Attach(toDelete); //attach        context.Destinations.Remove(toDelete);        context.SaveChanges();      }    }

自然就没有了先加载实体到内存中的sql,只有一个简单的删除sql。attach方法是让EF知道DestinationId为2的实体是一个存在的实体。当然不使用attach,直接调用Remove方法删除会报一个InvalidOperationException错:无法删除此对象,因为未在 ObjectStateManager 中找到它。
attach中文意为“连接,附上”等意思,后续还有很多attach方法高富帅的用法讲解。

还有一种不加载实体到内存就可以删除实体的简单方法,用EF直接执行sql:

    /// <summary>    /// 删除单个实体ExecuteSqlCommand    /// </summary>    private static void DeleteWineGlassBayExecuteSqlCommand()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        context.Database.ExecuteSqlCommand("delete from baga.Locations where LocationName = 'Hawaii'");      }    }

可见,都不需要调用上下文的SaveChanges方法了,因为是直接执行sql,所有并不需要EF跟踪任何状态然后提交到数据库。
ok,对于单个的增删改查就是这么简单,有Linq的写法,也有Lambda表达式的写法,都很简单,下面看复杂点的。

 

 

二、主从表关联数据的各种增删改查

 

1.增加
主从表数据的添加分为:仅添加从表数据、添加主表同时增加相关联的从表数据

仅添加从表数据:

    /// <summary>    /// 添加从表数据    /// </summary>    private static void NewGrandCanyonResort()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var resort = new DbContexts.Model.Resort        {          Name = "Pete's Luxury Resort"        };        context.Lodgings.Add(resort);        var canyon = (from d in context.Destinations               where d.Name == "Grand Canyon"               select d).Single();        canyon.Lodgings.Add(resort);        context.SaveChanges();      }    }

Lodging是住宿类,有两个类继承本类,分别Resort度假村类和Hostel宿舍类。上面的方法添加了一个Grand Canyon景点的度假村,Name是Pete's Luxury Resort。这里的Grand Canyon是主表数据,Pete's Luxury Resort是从表数据。跟踪到的sql:

exec sp_executesql N'insert [dbo].[Lodgings]([Name], [Owner], [MilesFromNearestAirport], [destination_id], [PrimaryContactId], [SecondaryContactId], [Entertainment], [Activities], [MaxPersonsPerRoom], [PrivateRoomsAvailable], [Discriminator])values (@0, null, @1, @2, null, null, null, null, null, null, @3)select [LodgingId]from [dbo].[Lodgings]where @@ROWCOUNT > 0 and [LodgingId] = scope_identity()',N'@0 nvarchar(200),@1 decimal(18,2),@2 int,@3 nvarchar(128)',@0=N'Pete''s Luxury Resort',@1=0,@2=1,@3=N'Resort'

这跟单个实体的增加区别就是这样增加出来的数据外键值是有的,不是一个独立的实体了,是一个指向主表某条数据的从表数据。并且注意看sql,最后一列是Discriminator,这个是用来区分继承类映射的列,告诉我们这个属于哪个表的数据,因为Resort度假村类跟Destination没有直接的关系,而是继承的住宿类Lodging跟Destination是多对一的关系。具体点这里了解。

添加主表数据同时添加相关联的从表数据:

    /// <summary>    /// 添加主表数据同时添加相关联的从表数据    /// </summary>    private static void AddSingleAndRelatedData()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var destination = new DbContexts.Model.Destination        {          Name = "AnHui HuangShan",          Lodgings = new List<DbContexts.Model.Lodging>          {             new DbContexts.Model.Lodging {Name="HuangShan Hotel"},            new DbContexts.Model.Lodging {Name="YingKeSong Hotel"}          }        };        context.Destinations.Add(destination);        context.SaveChanges();      }    }

监控到三段sql,分别是添加主表数据,和两条添加相关联的从表数据,它们是通过外键destination_id相关联的:

exec sp_executesql N'insert [baga].[Locations]([LocationName], [Country], [Description], [Photo], [TravelWarnings], [ClimateInfo])values (@0, null, null, null, null, null)select [LocationID]from [baga].[Locations]where @@ROWCOUNT > 0 and [LocationID] = scope_identity()',N'@0 nvarchar(200)',@0=N'AnHui HuangShan'
exec sp_executesql N'insert [dbo].[Lodgings]([Name], [Owner], [MilesFromNearestAirport], [destination_id], [PrimaryContactId], [SecondaryContactId], [Entertainment], [Activities], [MaxPersonsPerRoom], [PrivateRoomsAvailable], [Discriminator])values (@0, null, @1, @2, null, null, null, null, null, null, @3)select [LodgingId]from [dbo].[Lodgings]where @@ROWCOUNT > 0 and [LodgingId] = scope_identity()',N'@0 nvarchar(200),@1 decimal(18,2),@2 int,@3 nvarchar(128)',@0=N'HuangShan Hotel',@1=0,@2=5,@3=N'Lodging'
exec sp_executesql N'insert [dbo].[Lodgings]([Name], [Owner], [MilesFromNearestAirport], [destination_id], [PrimaryContactId], [SecondaryContactId], [Entertainment], [Activities], [MaxPersonsPerRoom], [PrivateRoomsAvailable], [Discriminator])values (@0, null, @1, @2, null, null, null, null, null, null, @3)select [LodgingId]from [dbo].[Lodgings]where @@ROWCOUNT > 0 and [LodgingId] = scope_identity()',N'@0 nvarchar(200),@1 decimal(18,2),@2 int,@3 nvarchar(128)',@0=N'YingKeSong Hotel',@1=0,@2=5,@3=N'Lodging'

注意看第一段sql,使用了scope_identity(),这个和ado.net里在每条insert的sql后加上;SELECT @@IDENTITY是一个意思,它会返回自增长的主键id。这里当然是需要返回主键id的,因为后面从表的数据需要用这个当外键。我们可以复制第一条sql到数据库环境里执行看看效果。

 

2.查找
根据主表找从表数据(显示加载:先Entry,然后Collection):

    /// <summary>    /// 根据主表找从表数据(显示加载)    /// </summary>    private static void LoadRelateData()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var canyon = (from d in context.Destinations               where d.Name == "Grand Canyon"               select d).Single();        context.Entry(canyon).Collection(d => d.Lodgings).Load(); //显示加载        foreach (var lodging in context.Lodgings.Local)  //遍历的是内存中Lodgings的数据        {          Console.WriteLine(lodging.Name);        }      }    }

根据从表找主表数据(显示加载:先Entry,然后Reference):

    /// <summary>    /// 根据从表找主表数据(显示加载)    /// </summary>    private static void LoadPrimaryKeyData()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var lodging = context.Lodgings.First();        //context.Entry(lodging).Reference(l => l.PrimaryContact).Load();        context.Entry(lodging).Reference(l => l.Destination).Load();        foreach (var destination in context.Destinations.Local)  //遍历的是内存中的Destinations数据        {          Console.WriteLine(destination.Name);        }      }    }

这是EF标准的查询关联表的数据。如果不看官方的API,我们会怎么根据主表取从表数据、根据从表取主表数据呢?我想是这样:先拿到主表主键id,然后根据id使用find方法(甚至使用ExcuteSqlCommad发送sql)去从表里查,最后得到结果集。从表查主表也一样。这样写有什么不好呢?语句多了不少,其次不是EF建议的写法,个人还是建议使用Entry配合Collection和Reference方法。

 

3.修改
修改从表的外键:

    /// <summary>    /// 修改从表的外键    /// </summary>    private static void ChangeLodgingDestination()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var hotel = (from l in context.Lodgings               where l.Name == "Grand Hotel"               select l).Single();        var reef = (from d in context.Destinations              where d.Name == "Great Barrier Reef"              select d).Single();        hotel.Destination = reef;        context.SaveChanges();      }    }

Grand Hotel本来的外键是LocationId为1的Grand Canyon,我们的代码把它修改成到了LocationId为4的Great Barrier Reef下。生成的sql简单明了:

exec sp_executesql N'update [dbo].[Lodgings]set [destination_id] = @0where ([LodgingId] = @1)',N'@0 int,@1 int',@0=4,@1=1

 

4.删除
删除分为:删除主从表关系、删除主表数据不删除相关联的从表数据、同时删除主从表数据(级联和不级联删除)、删除主表数据同时修改相关联的从表数据指向另一个主表实体

删除主从表关系:主从表的关系是通过从表的外键列确定的,我们只需要赋值从表外键列为null即可

    /// <summary>    /// 删除主从表关系(ForeignKeys方式)    /// </summary>    private static void RemovePrimaryContactForeignKeys()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var davesDump = (from l in context.Lodgings                 where l.Name == "Dave's Dump"                 select l).Single();        davesDump.PrimaryContactId = null;        context.SaveChanges();      }    }

另一种方式:

    /// <summary>    /// 删除主从表关系(Reference方式)    /// </summary>    private static void RemovePrimaryContactReference()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var davesDump = (from l in context.Lodgings                 where l.Name == "Dave's Dump"                 select l).Single();        context.Entry(davesDump).Reference(l => l.PrimaryContact).Load(); //找主表数据        davesDump.PrimaryContact = null; //清空        context.SaveChanges();      }    }

住宿类Lodging跟人类Person有一个多对一的关系,这个很好理解,一个人可以有多个酒店。Dave's Dump这个住宿的地方本来本来是PrimaryContactId为1,也就是PersionId为1的这个人的记录,上面的两个方法都是修改这个1为空,即这个Dave's Dump这个住宿的地方不属于任何人了。看看生成的sql:

exec sp_executesql N'update [dbo].[Lodgings]set [PrimaryContactId] = nullwhere ([LodgingId] = @0)',N'@0 int',@0=2

 

删除主表数据不删除相关联的从表数据:

ok,先介绍两个新的实体:

  /// <summary>  /// 旅行类  /// </summary>  public class Trip  {    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]    public Guid Identifier { get; set; }    public DateTime StartDate { get; set; }    public DateTime EndDate { get; set; }    public string Description { get; set; }    public decimal CostUSD { get; set; }    [Timestamp]    public byte[] RowVersion { get; set; }    public int DestinationId { get; set; }    [Required]    public Destination Destination { get; set; }    public List<Activity> Activities { get; set; }  }
  /// <summary>  /// 预约类  /// </summary>  public class Reservation  {    public Reservation()    {      Payments = new List<Payment>();    }    public int ReservationId { get; set; }    public DateTime DateTimeMade { get; set; }    public Person Traveler { get; set; }    public Trip Trip { get; set; }    public Nullable<DateTime> PaidInFull { get; set; }    public List<Payment> Payments { get; set; }  }

没有配置任何Data Annotation和Fluent API。两个实体的关系是通过Reservation类的Trip导航属性确立的。很明显,这是一个一对一的关系,且预约类Reservation的外键Trip_Identifier是可空的(为何生成的外键名是Trip_Identifier?EF默认映射是取主表实体类名字加主键列),意思很明确,就是预约表Reservations的数据可以对应到旅行表Trip,也可以不对应:

我们在两张表任何一张上右键 - 关系查看下这个主外键关系的设置。删除规则为:不执行任何操作。意思是删除主表数据,对应的从表数据不会被删除;如果打开了级联删除,那么发送一条删除主表数据的sql到数据库,数据库不仅会删除主表数据,也会自动删除对应的从表数据。如果使用Fluent API配置这个一对一的关系,那么默认会打开级联删除。要了解更多级联删除的知识请点击

看看这两张表在数据库里有的数据:

很明显,Reservations预约表的Trip_Identifier列(guid类型)指向了Trips表的主键列Identifier。我们试着删除:

    /// <summary>    /// 不加载从表数据直接删除主表数据    /// </summary>    private static void DeleteTrip()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var trip = (from t in context.Trip              where t.Description == "Trip from the database"              select t).Single();        context.Trip.Remove(trip);        context.SaveChanges();      }    }

根据Description列的内容从数据库取出主表Trips的某条数据,然后直接调用上下文的Remove方法删除。程序跑起来会报一个DbUpdateException错:

违反了主外键的约束。这个很好理解:从表的某条数据指向主表的这条数据,主表的这条数据自然不能随便删除。我们修改下方法,删除主表某条数据,同时加载其关联的从表数据:

    /// <summary>    /// 同时加载从表数据    /// </summary>    private static void DeleteTripLoadRelateData()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var trip = (from t in context.Trip              where t.Description == "Trip from the database"              select t).Single();        var res = (from r in context.Reservations              where r.Trip.Description == "Trip from the database"              select r).Single();        context.Trip.Remove(trip);        context.SaveChanges();      }    }

看看这几行代码生成了多少sql:
1.查出主表数据:

SELECT TOP (2) [Extent1].[Identifier] AS [Identifier], [Extent1].[StartDate] AS [StartDate], [Extent1].[EndDate] AS [EndDate], [Extent1].[Description] AS [Description], [Extent1].[CostUSD] AS [CostUSD], [Extent1].[RowVersion] AS [RowVersion], [Extent1].[DestinationId] AS [DestinationId]FROM [dbo].[Trips] AS [Extent1]WHERE N'Trip from the database' = [Extent1].[Description]

2.查出从表数据:

SELECT TOP (2) [Extent1].[ReservationId] AS [ReservationId], [Extent1].[DateTimeMade] AS [DateTimeMade], [Extent1].[PaidInFull] AS [PaidInFull], [Extent1].[Traveler_PersonId] AS [Traveler_PersonId], [Extent1].[Trip_Identifier] AS [Trip_Identifier]FROM [dbo].[Reservations] AS [Extent1]INNER JOIN [dbo].[Trips] AS [Extent2] ON [Extent1].[Trip_Identifier] = [Extent2].[Identifier]WHERE N'Trip from the database' = [Extent2].[Description]

3.更新从表的外键为null:

exec sp_executesql N'update [dbo].[Reservations]set [Trip_Identifier] = nullwhere (([ReservationId] = @0) and ([Trip_Identifier] = @1))',N'@0 int,@1 uniqueidentifier',@0=1,@1='D928B2FE-C667-49C2-BEAC-E9425A12F580'

4.删除主表数据:

exec sp_executesql N'delete [dbo].[Trips]where (([Identifier] = @0) and ([RowVersion] = @1))',N'@0 uniqueidentifier,@1 binary(8)',@0='D928B2FE-C667-49C2-BEAC-E9425A12F580',@1=0x00000000000007D1

看完了你肯定会想EF删除主表数据真麻烦:同时加载主表和从表的数据,然后设置从表外键为null让它不指向主表任何数据,然后再删除主表数据。
正常的思维删除主表数据是这样的:取出主表的主键字段,然后根据主键去从表里找,看看有没有相关联的数据,有就赋值外键为null,最后删除主表数据。写出来无非就是各种find,然后update,最后delete。这是正常的思维和写法,但是缺点很明显:比上面的方法多写了很多代码。
所以,我们还是按照EF的思路来:删除主表数据,就同时加载主表和从表数据到内存中再执行删除主表数据的操作。我们只需要直接调用Remove方法就好,EF自动帮我们把从表的相关数据外键列设置为null。

删除主表数据同时删除相关联的从表数据(级联删除)

    /// <summary>    /// 显示加载从表数据    /// </summary>    private static void DeleteGrandCanyonLoadRelateData()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var canyon = (from d in context.Destinations               where d.Name == "Grand Canyon"               select d).Single();        context.Entry(canyon).Collection(d => d.Lodgings).Load(); //显示加载        //不调用Load,也可以先调用Query方法,在内存中执行需要的操作再把结果集加载到内存中,效率!比如:        //context.Entry(canyon).Collection(d => d.Lodgings).Query().Where(l => l.Name.Contains("Hotel")).Load();        context.Destinations.Remove(canyon);        context.SaveChanges();      }    }

因为Destination类和Lodging类已经设置好了级联删除,所以直接找到主键删除即可,相关联的从表数据由数据库自动删除:

    /// <summary>    /// 级联删除:不加载从表数据(数据库里必须设置是级联删除)    /// </summary>    private static void DeleteGrandCanyonWithoutLoadRelateData()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var canyon = (from d in context.Destinations               where d.Name == "Grand Canyon"               select d).Single();        context.Destinations.Remove(canyon);        context.SaveChanges();      }    }

删除主表数据同时删除相关联的从表数据(非级联删除)

标注每个从表的数据为删除状态,然后调用数据库上下文的SaveChanges方法:

    /// <summary>    /// 普通删除:删除主表数据,同时标注从表数据为删除状态(数据库关闭了级联删除的情况,可以手动去数据库的外键关系修改,也可以Fluent API配置关闭级联删除)    /// </summary>    private static void DeleteGrandCanyonAndMarkChildEntitiesDeletion()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var canyon = (from d in context.Destinations               where d.Name == "Grand Canyon"               select d).Single();        foreach (var lodging in canyon.Lodgings.ToList())        {          context.Lodgings.Remove(lodging);  //先标记相关的从表数据为删除状态        }        context.Destinations.Remove(canyon);  //再标记主表数据为删除装填        context.SaveChanges();  //执行上面的所有标记      }    }

 

删除主表数据同时修改相关联的从表数据指向另一个主表实体:

    /// <summary>    /// 普通删除:删除主表数据,同时设置从表数据指向另一个主键(数据库默认打开关闭级联删除都可以)    /// </summary>    private static void DeleteGrandCanyonAndChangeChildEntitiesPrimaryKey()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        //找到要删除的主表数据        var canyon = (from d in context.Destinations               where d.Name == "Grand Canyon"               select d).Single();        //找到和主表数据相关的从表数据并修改其主键值,让这些相关的从表数据指向另一个存在的主表数据        var hawaii = context.Destinations.Find(2);  //hawaii此时在数据库的主键是2(find方法生成的sql稍复杂,建议使用下面的普通写法)        //var hawaii = (from d in context.Destinations        //       where d.DestinationId == 2        //       select d).Single();        foreach (var lodging in canyon.Lodgings.ToList())        {          lodging.Destination = hawaii;        }        //最后删除主表数据,可以此时只是单独的删除主表数据,它已经没有了相关的从表数据了        context.Destinations.Remove(canyon);        context.SaveChanges();      }    }

 

 

补充内容:以上所有演示我们调用SaveChanges都是提交一个更改,我们试着提交多个操作:

    /// <summary>    /// 一次提交多个修改    /// </summary>    private static void MakeMultipleChanges()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var niagaraFalls = new DbContexts.Model.Destination        {          Name = "Niagara Falls",          Country = "USA"        };        context.Destinations.Add(niagaraFalls);        var wineGlassBay = (from d in context.Destinations                  where d.Name == "Wine Glass Bay"                  select d).Single();        wineGlassBay.Description = "Picturesque bay with beaches";        context.SaveChanges();      }    }

增加一个Destinations表对象,又修改了一个对象,跟踪下sql发现很明确的是一条insert,一条update的sql。SaveChanges也是一个事务,如果一个不成功,那么所有都提交不成功。

 

仔细看上面的DbSet.Add方法可知,DbSet.Add方法返回的对象就是我们添加的实体对象,上面的Add方法返回的就是DbContexts.Model.Destination。这个给我们编码提供了很好的便利性,我们看一个方法:

    /// <summary>    /// 有就查询,没有就添加并查询    /// </summary>    private static void FindOrAddPerson()    {      using (var context = new DbContexts.DataAccess.BreakAwayContext())      {        var ssn = 123456789;        var person = context.People.Find(ssn) ?? context.People.Add(new DbContexts.Model.Person        {          SocialSecurityNumber = ssn,          FirstName = "Phelps",          LastName = "Michael"        });        Console.WriteLine(person.FirstName);      }    }

??表示前者如果为null就使用后者。很明显,库里不存在ssn为123456789的人,那么程序添加一个新的ssn为123456789的人,添加完毕,这个person对象就是我们刚调用Add方法添加的person。这里并没有调用SaveChanges方法,如果调用SaveChanges方法通过调用person.PersonId还可以获取自增长的主键id。