注:本章节多次演示了各种删除,若要重复查看效果,需解开注释初始化数据的方法。

一、EF对单个实体的增查改删

增加单个实体:

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        /// <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();
            }
        }
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产生的insert sql:

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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'
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查询单个实体:

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        /// <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);
            }
        }
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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):

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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
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修改单个实体:

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        /// <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();
            }
        }
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产生的update sql:

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

删除单个实体:

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        /// <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();
            }
        }
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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。使用attach方法改进:

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        /// <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();
            }
        }
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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、贴上”等意思。
注:Attach的实体事先不能已经在内存中,否则上下文会追踪到两个相同键名的实体,并且会报一个InvalidOperationException错:An object with the same key already exists in the ObjectStateManager.

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:

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        /// <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'");
            }
        }
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可见,都不需要调用上下文的SaveChanges方法了,因为是直接执行sql,所以并不需要被数据库上下文跟踪到任何状态。
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:

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        /// <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();
            }
        }
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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:

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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'
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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,是用来区分属于哪个表的数据,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:

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        /// <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();
            }
        }
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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:

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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'
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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'
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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'
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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。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):
2013.09.04修改:之前描述的“根据主表找从表数据”有错误,Collection不是主表找从表,而是找导航属性是一个集合的。

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        /// <summary>
        /// 查找导航属性为一个集合的
        /// </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);
                }
            }
        }
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根据从表找主表数据(显示加载:先Entry,然后Reference):
2013.09.04修改:之前描述的“根据从表找主表数据”有错误,Reference不仅仅是根据从表找主表,而是找导航属性是一个实体对象的,常用于一对多关系里从表对象找主表对象,也可以用于一对一关系的查找。

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        /// <summary>
        /// 查找导航属性为一个实体对象的
        /// </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);
                }
            }
        }
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这是EF标准的查询关联表的数据。如果不看官方的API,大家会怎么查呢?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,我个人还是建议使用Entry配合Collection和Reference方法。

3.修改
修改从表的外键:

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        /// <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();
            }
        }
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Grand Hotel本来的外键是LocationId为1的Grand Canyon,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] = @0
where ([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,只需要赋值从表外键列为null即可

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        /// <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();
            }
        }
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另一种方式:

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        /// <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();
            }
        }
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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] = null
where ([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:

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    /// <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; }
    }
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    /// <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; }
    }
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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:

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。使用Optional系列方法配置的关系不会打开级联删除。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。试着删除:

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        /// <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();
            }
        }
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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。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:

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        /// <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();
            }
        }
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看看这几行代码生成了多少sql:
1.查出主表数据:

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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]
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2.查出从表数据:

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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]
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3.更新从表的外键为null:

exec sp_executesql N'update [dbo].[Reservations]
set [Trip_Identifier] = null
where (([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,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。只需要直接调用Remove方法就好,EF自动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)

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        /// <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();
            }
        }
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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:

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        /// <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();
            }
        }
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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:

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        /// <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
            }
        }
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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:

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        /// <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();
            }
        }
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补充内容:以上所有的演示调用SaveChanges都是提交一个更改,试着提交多个操作:

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        /// <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();
            }
        }
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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添加的实体对象,上面的Add方法返回的就是DbContexts.Model.Destination。这个给编码提供了很好的便利性,来看一个方法:

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        /// <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);
            }
        }
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??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对象就是刚刚调用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。详细了解请点园长dudu的文章

感谢耐心读完我的长文,希望你能从中有所感悟。本文源码

EF DbContext 系列文章导航
  1. EF如何操作内存中的数据和加载外键数据:延迟加载、贪婪加载、显示加载  本章源码
  2. 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  本章源码
  3. 使用EF自带的EntityState枚举和自定义枚举实现单个和多个实体的增删改查  本章源码
  4. EF里查看/修改实体的当前值、原始值和数据库值以及重写SaveChanges方法记录实体状态  本章源码
  5. EF里如何定制实体的验证规则和实现IObjectWithState接口进行验证以及多个实体的同时验证  本章源码
  6. 重写ValidateEntity虚方法实现可控的上下文验证和自定义验证  本章源码
 
 http://www.cnblogs.com/oppoic/p/ef_single_entity_crud_related_data_crud.html