EF里查看/修改實(shí)體的當(dāng)前值、原始值和數(shù)據(jù)庫(kù)值以及重寫SaveChanges方法記錄實(shí)體狀態(tài) 原文:EF里查看/修改實(shí)體的當(dāng)前值、原始值和數(shù)據(jù)庫(kù)值以及重寫SaveChanges方法記錄實(shí)體狀態(tài)

本文目錄

• 查看實(shí)體當(dāng)前、原始和數(shù)據(jù)庫(kù)值：DbEntityEntry
• 查看實(shí)體的某個(gè)屬性值：GetValue<TValue>方法
• 拷貝DbPropertyValues到實(shí)體：ToObject方法
• 修改DbPropertyValues當(dāng)前值：索引器
• 克隆實(shí)體：Clone方法
• 設(shè)置實(shí)體的值：SetValues方法
• 克隆實(shí)體：SetValues
• 獲取和設(shè)置實(shí)體的單個(gè)屬性：Property方法
• 查詢實(shí)體的屬性是否被修改：IsModified方法
• 修改導(dǎo)航屬性
• 重新加載實(shí)體：Reload方法
• 讀取相關(guān)聯(lián)的實(shí)體和狀態(tài)：DbContext.ChangeTracker.Entries方法
• EF里如何解決更新時(shí)的沖突
• 重寫上下文的SaveChanges方法記錄結(jié)果集里實(shí)體的各種增/刪/改
• 本文源碼和系列文章導(dǎo)航

文章開(kāi)始前建議大家為了更好的記憶最好自己實(shí)現(xiàn)文中的所有方法。如果非要直接運(yùn)行我的demo，必要的時(shí)候需要恢復(fù)下數(shù)據(jù)庫(kù)數(shù)據(jù)，否則找不到記錄。

之前的章節(jié)已經(jīng)演示了context.Entry方法可以拿到實(shí)體的狀態(tài)（EntityState），來(lái)看一個(gè)方法：

/// <summary>/// 單個(gè)實(shí)體的狀態(tài)/// </summary>private static void PrintState(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var canyon = (from d in context.Destinationswhere d.Name == "Grand Canyon"select d).Single();DbEntityEntry<DbContexts.Model.Destination> entry = context.Entry(canyon);Console.WriteLine("Before Edit：{0}", entry.State); //Unchaged canyon.TravelWarnings = "Take a lot of Water!";DbEntityEntry<DbContexts.Model.Destination> entrys = context.Entry(canyon);
Console.WriteLine("After Edit：{0}", entrys.State); //Modified }}

context.Entry方法有兩個(gè)重載，分別返回泛型DbEntityEntry<TEntity>和非泛型的DbEntityEntry，它們都可以監(jiān)測(cè)到實(shí)體的狀態(tài)，并且通過(guò)DbEntityEntry還可以操作實(shí)體的當(dāng)前值、原始值和數(shù)據(jù)庫(kù)值。分別是：

• 當(dāng)前值（Current Value）：程序里設(shè)置實(shí)體屬性的值（在內(nèi)存中，還沒(méi)提交數(shù)據(jù)庫(kù)）；
• 原始值（Original Value）：被數(shù)據(jù)庫(kù)上下文跟蹤到時(shí)的值（程序取出數(shù)據(jù)庫(kù)的值，可能不是最新的）；
• 數(shù)據(jù)庫(kù)值（Database Value）：數(shù)據(jù)庫(kù)里的值（此時(shí)此刻數(shù)據(jù)庫(kù)里最新的值）

來(lái)看一個(gè)例子：

/// <summary>/// 打印實(shí)體當(dāng)前、原始和數(shù)據(jù)庫(kù)值/// </summary>private static void PrintLodgingInfo(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var hotel = (from d in context.Lodgingswhere d.Name == "Grand Hotel"select d).Single();hotel.Name = "Super Grand Hotel";context.Database.ExecuteSqlCommand(@"UPDATE Lodgings SET Name = 'Not-So-Grand Hotel' WHERE Name = 'Grand Hotel'");PrintChangeTrackingInfo(context, hotel);}}private static void PrintChangeTrackingInfo(DbContexts.DataAccess.BreakAwayContext context, DbContexts.Model.Lodging entity){var entry = context.Entry(entity);Console.WriteLine(entry.Entity.Name);Console.WriteLine("State: {0}", entry.State);Console.WriteLine("\nCurrent Values:");PrintPropertyValues(entry.CurrentValues);Console.WriteLine("\nOriginal Values:");PrintPropertyValues(entry.OriginalValues);Console.WriteLine("\nDatabase Values:");PrintPropertyValues(entry.GetDatabaseValues());}private static void PrintPropertyValues(DbPropertyValues values){foreach (var propertyName in values.PropertyNames){Console.WriteLine(" - {0}: {1}", propertyName, values[propertyName]);}}

方法分析：先從數(shù)據(jù)庫(kù)取出一個(gè)實(shí)體，然后修改其Name屬性，這個(gè)時(shí)候當(dāng)前值（Current）和原始值（Original）都有了，分別是：修改后的值（還沒(méi)提交，在內(nèi)存中）和從庫(kù)里取出來(lái)時(shí)實(shí)體的值。再使用Database.ExecuteSqlCommand執(zhí)行了一段修改此對(duì)象在數(shù)據(jù)庫(kù)中的值，這個(gè)時(shí)候數(shù)據(jù)庫(kù)值（Database）也有了變化，這個(gè)實(shí)體的三個(gè)值都不相同了。還沒(méi)看到打印結(jié)果，在執(zhí)行entry.GetDatabaseValues()方法時(shí)報(bào)了一個(gè)EntitySqlException錯(cuò)：

找不到類型DbContexts.DataAccess.Lodging，項(xiàng)目的Lodging實(shí)體明明在DbContexts.Model.Lodging命名空間下，反復(fù)檢查代碼沒(méi)發(fā)現(xiàn)任何問(wèn)題，報(bào)這個(gè)錯(cuò)真是很疑惑。最后通過(guò)搜索引擎才知道這是EF4.1/4.2版本的一個(gè)bug，解決辦法：修改實(shí)體和上下文到一個(gè)命名空間，或者使用EF4.3 release。看看本書作者Julie Lerman在msdn論壇上關(guān)于此bug的回復(fù)

換成4.3版本的EF問(wèn)題就立馬解決了（源碼的libs目錄下提供了EF4.3）。看下打印的結(jié)果：

結(jié)果分析：當(dāng)前值為方法里修改的值、原始值是從數(shù)據(jù)庫(kù)取出未做任何操作的值、數(shù)據(jù)庫(kù)值是此時(shí)數(shù)據(jù)庫(kù)里的值。當(dāng)然新添加的實(shí)體不會(huì)有原始值和數(shù)據(jù)庫(kù)值、刪除的實(shí)體也不會(huì)有當(dāng)前值，利用EntityState完善下方法：

private static void PrintChangeTrackingInfo(DbContexts.DataAccess.BreakAwayContext context, DbContexts.Model.Lodging entity){var entry = context.Entry(entity);Console.WriteLine(entry.Entity.Name);Console.WriteLine("State: {0}", entry.State);if (entry.State != EntityState.Deleted) //標(biāo)記刪除的實(shí)體不會(huì)有當(dāng)前值 {Console.WriteLine("\nCurrent Values:");PrintPropertyValues(entry.CurrentValues);}if (entry.State != EntityState.Added) //新添加的時(shí)候不會(huì)有原始值和數(shù)據(jù)庫(kù)值 {Console.WriteLine("\nOriginal Values:");PrintPropertyValues(entry.OriginalValues);Console.WriteLine("\nDatabase Values:");PrintPropertyValues(entry.GetDatabaseValues());}}

為了測(cè)試重寫下PrintLodgingInfo方法：

/// <summary>/// 測(cè)試打印添加和刪除時(shí)實(shí)體當(dāng)前、原始和數(shù)據(jù)庫(kù)值/// </summary>private static void PrintLodgingInfoAddAndDelete(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var hotel = (from d in context.Lodgingswhere d.Name == "Grand Hotel"select d).Single();PrintChangeTrackingInfo(context, hotel); //默認(rèn)var davesDump = (from d in context.Lodgingswhere d.Name == "Dave's Dump"select d).Single();context.Lodgings.Remove(davesDump);PrintChangeTrackingInfo(context, davesDump); //測(cè)試刪除實(shí)體var newMotel = new DbContexts.Model.Lodging { Name = "New Motel" };context.Lodgings.Add(newMotel);PrintChangeTrackingInfo(context, newMotel); //測(cè)試新添加實(shí)體 }}

當(dāng)然上面打印實(shí)體類型的方法并不通用，修改第二個(gè)參數(shù)為object類型：

/// <summary>/// 通用的打印實(shí)體方法/// </summary>private static void PrintChangeTrackingInfo(DbContexts.DataAccess.BreakAwayContext context, object entity){var entry = context.Entry(entity);Console.WriteLine("Type：{0}", entry.Entity.GetType()); //打印實(shí)體類型Console.WriteLine("State: {0}", entry.State);if (entry.State != EntityState.Deleted) //標(biāo)記刪除的實(shí)體不會(huì)有當(dāng)前值 {Console.WriteLine("\nCurrent Values:");PrintPropertyValues(entry.CurrentValues);}if (entry.State != EntityState.Added) //新添加的時(shí)候不會(huì)有原始值和數(shù)據(jù)庫(kù)值 {Console.WriteLine("\nOriginal Values:");PrintPropertyValues(entry.OriginalValues);Console.WriteLine("\nDatabase Values:");PrintPropertyValues(entry.GetDatabaseValues());}}

看看打印結(jié)果：

之前打印實(shí)體的各種屬性都是通過(guò)遍歷的形式（PrintPropertyValues方法）打印出來(lái)，如果僅取某個(gè)字段當(dāng)然沒(méi)必要這么麻煩，可以使用GetValue<TValue>：

/// <summary>/// 打印實(shí)體單個(gè)屬性/// </summary>private static void PrintOriginalName(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var hotel = (from d in context.Lodgingswhere d.Name == "Grand Hotel"select d).Single();hotel.Name = "Super Grand Hotel";string originalName = context.Entry(hotel).OriginalValues.GetValue<string>("Name");Console.WriteLine("Current Name: {0}", hotel.Name); //Super Grand HotelConsole.WriteLine("Original Name: {0}", originalName); //Grand Hotel}}

拷貝DbPropertyValues到實(shí)體：ToObject方法

/// <summary>/// 拷貝DbPropertyValues到實(shí)體：ToObject方法/// </summary>private static void TestPrintDestination(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var reef = (from d in context.Destinationswhere d.Name == "Great Barrier Reef"select d).Single();reef.TravelWarnings = "Watch out for sharks!";Console.WriteLine("Current Values");PrintDestination(reef);Console.WriteLine("\nDatabase Values");DbPropertyValues dbValues = context.Entry(reef).GetDatabaseValues();PrintDestination((DbContexts.Model.Destination)dbValues.ToObject()); //ToObject方法創(chuàng)建Destination實(shí)例 }}private static void PrintDestination(DbContexts.Model.Destination destination){Console.WriteLine("-- {0}, {1} --", destination.Name, destination.Country);Console.WriteLine(destination.Description);if (destination.TravelWarnings != null){Console.WriteLine("WARNINGS!: {0}", destination.TravelWarnings);}}

方法分析：從Destination表里取出Name為Great Barrier Reef的實(shí)體并修改其TravelWarnings字段，然后調(diào)用PrintDestination方法打印當(dāng)前實(shí)體的各屬性，再查出此實(shí)體在數(shù)據(jù)庫(kù)里的值，并且通過(guò)ToObject方法把數(shù)據(jù)庫(kù)取出來(lái)的這個(gè)對(duì)象也轉(zhuǎn)換成了實(shí)體對(duì)象。這么轉(zhuǎn)有什么好處呢？這個(gè)通過(guò)ToObject轉(zhuǎn)換的Destination實(shí)例不會(huì)被數(shù)據(jù)庫(kù)上下文追蹤，所以對(duì)其做的任何改變都不會(huì)提交數(shù)據(jù)庫(kù)。看看打印結(jié)果：

修改DbPropertyValues當(dāng)前值：

調(diào)用上下文的Entry方法，傳入要操作的實(shí)體對(duì)象，再打點(diǎn)就可以拿到實(shí)體的當(dāng)前值（CurrentValues）、原始值（OriginalValues）、數(shù)據(jù)庫(kù)值（GetDatabaseValues()），返回類型是DbPropertyValues，直接遍歷就可以輸出實(shí)體的所有屬性。當(dāng)然DbPropertyValues并不是只讀的。寫個(gè)方法修改試試：

/// <summary>/// 修改DbPropertyValues當(dāng)前值/// </summary>private static void ChangeCurrentValue(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var hotel = (from d in context.Lodgingswhere d.Name == "Grand Hotel"select d).Single();context.Entry(hotel).CurrentValues["Name"] = "Hotel Pretentious";Console.WriteLine("Property Value: {0}", hotel.Name);Console.WriteLine("State: {0}", context.Entry(hotel).State); //Modified }}

類似于索引器的方式賦值即可，賦值后實(shí)體的狀態(tài)已經(jīng)是Modified了，顯然已經(jīng)被上下文追蹤到了，這個(gè)時(shí)候調(diào)用上下文的SaveChanges方法將會(huì)提交到數(shù)據(jù)庫(kù)。那么如果只是想打印和修改實(shí)體狀態(tài)以供查看，并不像被提交到數(shù)據(jù)庫(kù)怎么辦？

最好的辦法就是克隆，先克隆實(shí)體然后操作克隆之后的實(shí)體：

/// <summary>/// 克隆實(shí)體：Clone/// </summary>private static void CloneCurrentValues(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var hotel = (from d in context.Lodgingswhere d.Name == "Grand Hotel"select d).Single();var values = context.Entry(hotel).CurrentValues.Clone(); //Clone方法values["Name"] = "Simple Hotel";Console.WriteLine("Property Value: {0}", hotel.Name);Console.WriteLine("State: {0}", context.Entry(hotel).State); //Unchanged }}

設(shè)置實(shí)體的值：SetValues方法

當(dāng)然實(shí)體的當(dāng)前值、原始值和數(shù)據(jù)庫(kù)值都是可以相互復(fù)制的：

/// <summary>/// 設(shè)置實(shí)體的值：SetValues方法/// </summary>private static void UndoEdits(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var canyon = (from d in context.Destinationswhere d.Name == "Grand Canyon"select d).Single();canyon.Name = "Bigger & Better Canyon";var entry = context.Entry(canyon);entry.CurrentValues.SetValues(entry.OriginalValues);entry.State = EntityState.Unchanged; //標(biāo)記未修改 Console.WriteLine("Name: {0}", canyon.Name); //Grand Canyon}}

上面的方法演示了拷貝原始值到當(dāng)前值，最終保存的是當(dāng)前值。很方便，不需要挨個(gè)賦值。

再看看如何使用SetValues方法實(shí)現(xiàn)之前說(shuō)的克隆實(shí)體：

/// <summary>/// 克隆實(shí)體：SetValues/// </summary>private static void CreateDavesCampsite(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var davesDump = (from d in context.Lodgingswhere d.Name == "Dave's Dump"select d).Single();var clone = new DbContexts.Model.Lodging();context.Lodgings.Add(clone);context.Entry(clone).CurrentValues.SetValues(davesDump); //克隆davesDump的值到新對(duì)象clone里clone.Name = "Dave's Camp"; //修改Name屬性context.SaveChanges(); //最后提交修改 Console.WriteLine("Name: {0}", clone.Name); //Dave's CampConsole.WriteLine("Miles: {0}", clone.MilesFromNearestAirport); //32.65Console.WriteLine("Contact Id: {0}", clone.PrimaryContactId); //1}} exec sp_executesql N'insert [dbo].[Lodgings]([Name], [Owner], [MilesFromNearestAirport], [destination_id], [PrimaryContactId], [SecondaryContactId], [Entertainment], [Activities], [MaxPersonsPerRoom], [PrivateRoomsAvailable], [Discriminator]) values (@0, null, @1, @2, @3, null, null, null, null, null, @4) select [LodgingId] from [dbo].[Lodgings] where @@ROWCOUNT > 0 and [LodgingId] = scope_identity()',N'@0 nvarchar(200),@1 decimal(18,2),@2 int,@3 int,@4 nvarchar(128)',@0=N'Dave''s Camp',@1=32.65,@2=1,@3=1,@4=N'Lodging'

很明顯實(shí)體已經(jīng)被克隆了。

獲取和設(shè)置實(shí)體的單個(gè)屬性：Property方法

/// <summary>/// 獲取和設(shè)置實(shí)體的單個(gè)屬性：Property方法/// </summary>private static void WorkingWithPropertyMethod(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var davesDump = (from d in context.Lodgingswhere d.Name == "Dave's Dump"select d).Single();var entry = context.Entry(davesDump);entry.Property(d => d.Name).CurrentValue = "Dave's Bargain Bungalows"; //設(shè)置Name屬性 Console.WriteLine("Current Value: {0}", entry.Property(d => d.Name).CurrentValue); //Dave's Bargain BungalowsConsole.WriteLine("Original Value: {0}", entry.Property(d => d.Name).OriginalValue); //Dave's DumpConsole.WriteLine("Modified？: {0}", entry.Property(d => d.Name).IsModified); //True }}

同樣可以查詢出實(shí)體的哪些屬性被修改了：IsModified方法

/// <summary>/// 查詢實(shí)體被修改字段：IsModified方法/// </summary>private static void FindModifiedProperties(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var canyon = (from d in context.Destinationswhere d.Name == "Grand Canyon"select d).Single();canyon.Name = "Super-Size Canyon";canyon.TravelWarnings = "Bigger than your brain can handle!!!";var entry = context.Entry(canyon);var propertyNames = entry.CurrentValues.PropertyNames; //獲取所有的Name列 IEnumerable<string> modifiedProperties = from name in propertyNameswhere entry.Property(name).IsModifiedselect name;foreach (var propertyName in modifiedProperties){Console.WriteLine(propertyName); //Name、TravelWarnings }}}

前面的章節(jié)已經(jīng)講解了如何查詢一對(duì)一、一對(duì)多等關(guān)系的導(dǎo)航屬性了，還不了解的點(diǎn)這里。現(xiàn)在講講如何修改導(dǎo)航屬性：

/// <summary>/// 修改導(dǎo)航屬性（Reference）：CurrentValue方法/// </summary>private static void WorkingWithReferenceMethod(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var davesDump = (from d in context.Lodgingswhere d.Name == "Dave's Dump"select d).Single();var entry = context.Entry(davesDump);entry.Reference(l => l.Destination).Load(); //顯示加載var canyon = davesDump.Destination;Console.WriteLine("Current Value After Load: {0}", entry.Reference(d => d.Destination).CurrentValue.Name);var reef = (from d in context.Destinationswhere d.Name == "Great Barrier Reef"select d).Single();entry.Reference(d => d.Destination).CurrentValue = reef; //修改Console.WriteLine("Current Value After Change: {0}", davesDump.Destination.Name);}}

打印結(jié)果：
Current Value After Load: Grand Canyon
Current Value After Change: Great Barrier Reef

注：上面的方法并沒(méi)有調(diào)用上下文的SaveChanges方法，故程序跑完數(shù)據(jù)也不會(huì)保存到數(shù)據(jù)庫(kù)，本文所有方法僅作演示都未提交數(shù)據(jù)庫(kù)。

有Reference找單個(gè)屬性的，那么自然也有Collection找集合屬性的：

/// <summary>/// 修改導(dǎo)航屬性（Collection）：CurrentValue方法/// </summary>private static void WorkingWithCollectionMethod(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var res = (from r in context.Reservationswhere r.Trip.Description == "Trip from the database"select r).Single();var entry = context.Entry(res);entry.Collection(r => r.Payments).Load();Console.WriteLine("Payments Before Add: {0}", entry.Collection(r => r.Payments).CurrentValue.Count);var payment = new DbContexts.Model.Payment { Amount = 245 };context.Payments.Add(payment);entry.Collection(r => r.Payments).CurrentValue.Add(payment); //修改Console.WriteLine("Payments After Add: {0}", entry.Collection(r => r.Payments).CurrentValue.Count);}}

打印結(jié)果：
Payments Before Add: 1
Payments After Add: 2

從數(shù)據(jù)庫(kù)取出實(shí)體加載到內(nèi)存中，可能并不立馬就展示給用戶看。在進(jìn)行一系列的排序、篩選等操作再展示出來(lái)。但是怎么確定展示的時(shí)候這些實(shí)體沒(méi)有被修改過(guò)呢？可以使用Reload方法重新加載：

/// <summary>/// 取當(dāng)前最新的數(shù)據(jù)庫(kù)值：Reload方法/// </summary>private static void ReloadLodging(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var hotel = (from d in context.Lodgingswhere d.Name == "Grand Hotel"select d).Single(); //取出實(shí)體context.Database.ExecuteSqlCommand(@"UPDATE dbo.Lodgings SET Name = 'Le Grand Hotel' WHERE Name = 'Grand Hotel'"); //立馬修改實(shí)體值（這個(gè)時(shí)候數(shù)據(jù)庫(kù)中的值已改變，但是取出來(lái)放在內(nèi)存中的值并沒(méi)改變）Console.WriteLine("Name Before Reload: {0}", hotel.Name);Console.WriteLine("State Before Reload: {0}", context.Entry(hotel).State);context.Entry(hotel).Reload();Console.WriteLine("Name After Reload: {0}", hotel.Name);Console.WriteLine("State After Reload: {0}", context.Entry(hotel).State);}}

打印結(jié)果：
Name Before Reload: Grand Hotel
State Before Reload: Unchanged
Name After Reload: Le Grand Hotel
State After Reload: Unchanged

可以看出Reload方法已經(jīng)重新取出了數(shù)據(jù)庫(kù)中的最新值。來(lái)看看Reload方法生成的sql：

SELECT [Extent1].[Discriminator] AS [Discriminator], [Extent1].[LodgingId] AS [LodgingId], [Extent1].[Name] AS [Name], [Extent1].[Owner] AS [Owner], [Extent1].[MilesFromNearestAirport] AS [MilesFromNearestAirport], [Extent1].[destination_id] AS [destination_id], [Extent1].[PrimaryContactId] AS [PrimaryContactId], [Extent1].[SecondaryContactId] AS [SecondaryContactId], [Extent1].[Entertainment] AS [Entertainment], [Extent1].[Activities] AS [Activities], [Extent1].[MaxPersonsPerRoom] AS [MaxPersonsPerRoom], [Extent1].[PrivateRoomsAvailable] AS [PrivateRoomsAvailable] FROM [dbo].[Lodgings] AS [Extent1] WHERE ([Extent1].[Discriminator] IN ('Resort','Hostel','Lodging')) AND ([Extent1].[LodgingId] = 1)

當(dāng)然Reload方法也會(huì)保存內(nèi)存中修改的數(shù)據(jù)，這個(gè)并不會(huì)沖突。在方法里的linq查詢后面加上：hotel.Name = "A New Name"; 打印結(jié)果就是這樣的了：
Name Before Reload: A New Name
State Before Reload: Modified
Name After Reload: Le Grand Hotel
State After Reload: Unchanged

注意，代碼里修改的Name已經(jīng)顯示了，并且標(biāo)記實(shí)體狀態(tài)為Modified了，Modified會(huì)在調(diào)用上下文的SaveChanges方法的時(shí)候提交到數(shù)據(jù)庫(kù)。這個(gè)過(guò)程是這樣的：

加載實(shí)體到內(nèi)存中 - 在內(nèi)存中對(duì)實(shí)體的某個(gè)屬性進(jìn)行修改 - 使用ExecuteSqlCommand方法執(zhí)行sql修改數(shù)據(jù)庫(kù)里該實(shí)體的值 - 調(diào)用Reload取出數(shù)據(jù)庫(kù)里本實(shí)體的最新值 - 調(diào)用SaveChanges方法的話，在內(nèi)存中對(duì)實(shí)體的修改也會(huì)被提交到數(shù)據(jù)庫(kù)

之前操作了單個(gè)實(shí)體，現(xiàn)在看看如何讀取關(guān)聯(lián)實(shí)體和狀態(tài)。使用DbContext.ChangeTracker.Entries方法：

　　　　 /// <summary>/// 讀取相關(guān)聯(lián)的實(shí)體和狀態(tài)：DbContext.ChangeTracker.Entries方法/// </summary>private static void PrintChangeTrackerEntries(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var res = (from r in context.Reservationswhere r.Trip.Description == "Trip from the database"select r).Single();context.Entry(res).Collection(r => r.Payments).Load();
res.Payments.Add(new DbContexts.Model.Payment { Amount = 245 });var entries = context.ChangeTracker.Entries();foreach (var entry in entries){Console.WriteLine("Entity Type: {0}", entry.Entity.GetType());Console.WriteLine(" - State: {0}", entry.State);}}}

添加了一個(gè)從表實(shí)體，并讀取所有關(guān)聯(lián)實(shí)體和其狀態(tài)，打印結(jié)果：
Entity Type: DbContexts.Model.Payment - State: Added
Entity Type: DbContexts.Model.Reservation - State: Unchanged
Entity Type: DbContexts.Model.Payment - State: Unchanged

EF里如何解決更新數(shù)據(jù)時(shí)的沖突

正常根據(jù)實(shí)體的主鍵修改實(shí)體的時(shí)候，EF是不會(huì)判斷數(shù)據(jù)修改之前有沒(méi)有被別的人修改過(guò)，但是如果做了并發(fā)控制，EF在更新某條記錄的時(shí)候才會(huì)拋錯(cuò)。這個(gè)系列文章的demo里有兩個(gè)實(shí)體做了并發(fā)控制：Person類的SocialSecurityNumber字段被標(biāo)記了ConcurrencyCheck；Trip類的RowVersion字段被標(biāo)記了Timestamp。來(lái)寫一個(gè)觸發(fā)DbUpdateConcurrencyException異常的方法并處理這個(gè)異常：

/// <summary>/// 修改實(shí)體/// </summary>private static void ConcurrencyDemo(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var trip = (from t in context.Trip.Include(t => t.Destination)where t.Description == "Trip from the database"select t).Single();trip.Description = "Getaway in Vermont";context.Database.ExecuteSqlCommand(@"UPDATE dbo.Trips SET CostUSD = 400 WHERE Description = 'Trip from the database'");SaveWithConcurrencyResolution(context);}}/// <summary>/// 嘗試保存/// </summary>private static void SaveWithConcurrencyResolution(DbContexts.DataAccess.BreakAwayContext context){try{context.SaveChanges();}catch (DbUpdateConcurrencyException ex){ResolveConcurrencyConflicts(ex);SaveWithConcurrencyResolution(context);}}

方法分析：取出實(shí)體 - 修改實(shí)體Description屬性（此時(shí)實(shí)體狀態(tài)為Modified）- 使用ExecuteSqlCommand執(zhí)行sql修改了CostUSD和Description字段（修改后時(shí)間戳已經(jīng)不同了，PS：使用ExecuteSqlCommand執(zhí)行sql不需要調(diào)用SaveChanges方法）- 調(diào)用上下文的SaveChanges方法保存之前被標(biāo)記為Modified的實(shí)體，這個(gè)時(shí)候就會(huì)報(bào)一個(gè)DbUpdateConcurrencyException的異常，因?yàn)闀r(shí)間戳列已經(jīng)找不到了，這個(gè)更新的where條件根本找不到記錄了。有時(shí)間戳的列更新都是雙條件，時(shí)間戳詳細(xì)用法點(diǎn)這里了解。

嘗試寫個(gè)方法解決這個(gè)沖突：

/// <summary>/// 解決沖突/// </summary>private static void ResolveConcurrencyConflicts(DbUpdateConcurrencyException ex){foreach (var entry in ex.Entries){Console.WriteLine("Concurrency conflict found for {0}", entry.Entity.GetType());Console.WriteLine("\nYou are trying to save the following values:");PrintPropertyValues(entry.CurrentValues); //用戶修改的值 Console.WriteLine("\nThe values before you started editing were:");PrintPropertyValues(entry.OriginalValues); //從庫(kù)里取出來(lái)時(shí)的值var databaseValues = entry.GetDatabaseValues(); //即時(shí)數(shù)據(jù)庫(kù)的值Console.WriteLine("\nAnother user has saved the following values:");PrintPropertyValues(databaseValues);Console.WriteLine("[S]ave your values, [D]iscard you changes or [M]erge?");var action = Console.ReadKey().KeyChar.ToString().ToUpper(); //讀取用戶輸入的字母switch (action){case "S":entry.OriginalValues.SetValues(databaseValues); //拷貝數(shù)據(jù)庫(kù)值到當(dāng)前值（恢復(fù)時(shí)間戳）break;case "D":entry.Reload(); //重新加載break;case "M":var mergedValues = MergeValues(entry.OriginalValues, entry.CurrentValues, databaseValues);//合并entry.OriginalValues.SetValues(databaseValues); //拷貝數(shù)據(jù)庫(kù)值到當(dāng)前值（恢復(fù)時(shí)間戳）entry.CurrentValues.SetValues(mergedValues); //拷貝合并后的值到當(dāng)前值，最終保存的是當(dāng)前值break;default:throw new ArgumentException("Invalid option");}}}

捕獲到異常后告知用戶要修改實(shí)體的原始值（用戶修改前從數(shù)據(jù)庫(kù)取出來(lái)的值）、現(xiàn)在的值（用戶修改的值）、數(shù)據(jù)庫(kù)里的值（此時(shí)數(shù)據(jù)庫(kù)里的值，這個(gè)值已被修改，不是用戶修改前取出來(lái)的值了），打印出來(lái)的結(jié)果顯示已經(jīng)有人修改了這條記錄了。最后是問(wèn)用戶是否保存修改。分別是保存、放棄、合并修改。

用戶輸入"S"表示“保存”，case語(yǔ)句塊里執(zhí)行的操作是拷貝數(shù)據(jù)庫(kù)值到原始值，這里該有疑惑了，調(diào)用SaveChanges方法保存的也是currentValues當(dāng)前值，跟databaseValues數(shù)據(jù)庫(kù)值還有OriginalValues原始值沒(méi)有任何關(guān)系啊。其實(shí)這么操作是恢復(fù)一下時(shí)間戳的值方便更新，之前說(shuō)過(guò)timestamp的列更新條件是兩個(gè)，任何一個(gè)不對(duì)都更新不了。看看sql：

exec sp_executesql N'update [dbo].[Trips] set [Description] = @0, [CostUSD] = @1 where (([Identifier] = @2) and ([RowVersion] = @3)) select [RowVersion] from [dbo].[Trips] where @@ROWCOUNT > 0 and [Identifier] = @2',N'@0 nvarchar(max) ,@1 decimal(18,2),@2 uniqueidentifier,@3 binary(8)',@0=N'Getaway in Vermont',@1=1000.00,@2='CF2E6BD3-7393-440C-941A- 9124C61CE04A',@3=0x00000000000007D2

結(jié)果只保存了自己的修改：

用戶輸入“D”表示“放棄”，case語(yǔ)句塊里執(zhí)行的是Reload方法，這個(gè)方法之前已經(jīng)介紹過(guò)了，是重新加載數(shù)據(jù)庫(kù)里的最新值（Latest Value）。恢復(fù)下數(shù)據(jù)庫(kù)數(shù)據(jù)再執(zhí)行下方法，看看sql：

SELECT [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 [Extent1].[Identifier] = cast('cf2e6bd3-7393-440c-941a-9124c61ce04a' as uniqueidentifier)

取了下數(shù)據(jù)庫(kù)里該實(shí)體最新的值（使用ExecuteSqlCommand更新后的值），沒(méi)有其他任何更新語(yǔ)句，就是放棄本次修改的意思，但是之前ExecuteSqlCommand方法執(zhí)行的修改是有效的，看看結(jié)果：

上面的“保存修改”和“放棄修改”只能保存一個(gè)，如果讓用戶修改的和ExecuteSqlCommand的修改同時(shí)生效呢，選擇M，意為合并。看看合并方法：

/// <summary>/// 合并/// </summary>private static DbPropertyValues MergeValues(DbPropertyValues original, DbPropertyValues current, DbPropertyValues database){var result = original.Clone(); //拷貝原始值并存放合并后的值foreach (var propertyName in original.PropertyNames) //遍歷原始值的所有列 {if (original[propertyName] is DbPropertyValues) //判斷當(dāng)前列是否復(fù)雜類型（很少） {var mergedComplexValues =MergeValues((DbPropertyValues)original[propertyName],(DbPropertyValues)current[propertyName],(DbPropertyValues)database[propertyName]); //是復(fù)雜類型的話就使用遞歸合并復(fù)雜類型的值((DbPropertyValues)result[propertyName]).SetValues(mergedComplexValues);}else //是普通里的話就和當(dāng)前值、數(shù)據(jù)庫(kù)值、原始值各種對(duì)比。修改了就賦值{if (!object.Equals(current[propertyName], original[propertyName]))result[propertyName] = current[propertyName];else if (!object.Equals(database[propertyName], original[propertyName]))result[propertyName] = database[propertyName];}}return result;}

看看sql：

exec sp_executesql N'update [dbo].[Trips] set [Description] = @0, [CostUSD] = @1 where (([Identifier] = @2) and ([RowVersion] = @3)) select [RowVersion] from [dbo].[Trips] where @@ROWCOUNT > 0 and [Identifier] = @2',N'@0 nvarchar(max) ,@1 decimal(18,2),@2 uniqueidentifier,@3 binary(8)',@0=N'Getaway in Vermont',@1=400.00,@2='CF2E6BD3-7393-440C-941A-9124C61CE04A',@3=0x00000000000007DC

看看結(jié)果：

用戶修改和ExecuteSqlCommand修改的都保存上了。

最后講一個(gè)更實(shí)用的東西：重寫上下文的SaveChanges方法記錄結(jié)果集里實(shí)體的各種增/刪/改。
先到BreakAwayContext類里添加一個(gè)屬性標(biāo)識(shí)使用數(shù)據(jù)庫(kù)上下文的SaveChanges方法還是使用自定義的SaveChanges方法：public bool LogChangesDuringSave { get; set; }

來(lái)看一個(gè)方法：

/// <summary>/// 記錄結(jié)果集的各種：增 / 刪 /改/// </summary>private static void TestSaveLogging(){using (var context = new DbContexts.DataAccess.BreakAwayContext()){var canyon = (from d in context.Destinationswhere d.Name == "Grand Canyon"select d).Single();//加載主表數(shù)據(jù) context.Entry(canyon).Collection(d => d.Lodgings).Load();//顯示加載出從表相關(guān)數(shù)據(jù)canyon.TravelWarnings = "Take a hat!";//修改主表字段context.Lodgings.Remove(canyon.Lodgings.First());//刪除相關(guān)聯(lián)從表的第一條數(shù)據(jù)context.Destinations.Add(new DbContexts.Model.Destination { Name = "Seattle, WA" });//添加一條主表數(shù)據(jù)context.LogChangesDuringSave = true; //設(shè)置標(biāo)識(shí)，使用自定義的SaveChanges方法 context.SaveChanges();}}

增加、修改、刪除操作等都有。運(yùn)行這個(gè)方法前需要在BreakAwayContext類里添加記錄的幫助類方法：

/// <summary>/// 記錄幫助類方法/// </summary>private void PrintPropertyValues(DbPropertyValues values, IEnumerable<string> propertiesToPrint, int indent = 1){foreach (var propertyName in propertiesToPrint){var value = values[propertyName];if (value is DbPropertyValues){Console.WriteLine("{0}- Complex Property: {1}", string.Empty.PadLeft(indent), propertyName);var complexPropertyValues = (DbPropertyValues)value;PrintPropertyValues(complexPropertyValues, complexPropertyValues.PropertyNames, indent + 1);}else{Console.WriteLine("{0}- {1}: {2}", string.Empty.PadLeft(indent), propertyName, values[propertyName]);}}}private IEnumerable<string> GetKeyPropertyNames(object entity){var objectContext = ((IObjectContextAdapter)this).ObjectContext;return objectContext.ObjectStateManager.GetObjectStateEntry(entity).EntityKey.EntityKeyValues.Select(k => k.Key);}

再在BreakAwayContext類里重寫下上下文的SaveChanges方法：

/// <summary>/// 重寫SaveChanges方法/// </summary>public override int SaveChanges(){if (LogChangesDuringSave) //根據(jù)表示判斷用重寫的SaveChanges方法，還是普通的上下文SaveChanges方法 {var entries = from e in this.ChangeTracker.Entries()where e.State != EntityState.Unchangedselect e; //過(guò)濾所有修改了的實(shí)體，包括：增加 / 修改 / 刪除foreach (var entry in entries){switch (entry.State){case EntityState.Added:Console.WriteLine("Adding a {0}", entry.Entity.GetType());PrintPropertyValues(entry.CurrentValues, entry.CurrentValues.PropertyNames);break;case EntityState.Deleted:Console.WriteLine("Deleting a {0}", entry.Entity.GetType());PrintPropertyValues(entry.OriginalValues, GetKeyPropertyNames(entry.Entity));break;case EntityState.Modified:Console.WriteLine("Modifying a {0}", entry.Entity.GetType());var modifiedPropertyNames = from n in entry.CurrentValues.PropertyNameswhere entry.Property(n).IsModifiedselect n;PrintPropertyValues(entry.CurrentValues, GetKeyPropertyNames(entry.Entity).Concat(modifiedPropertyNames));break;}}}return base.SaveChanges(); //返回普通的上下文SaveChanges方法}

運(yùn)行結(jié)果為：

所有添加/修改/刪除都記錄下來(lái)了，這個(gè)可以方便我們?cè)趯懗绦虻臅r(shí)候做更細(xì)微的控制，畢竟EF對(duì)實(shí)體操作的依據(jù)就是實(shí)體的各種狀態(tài)。

本文源碼

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posted on 2014-02-25 19:42 NET未來(lái)之路 閱讀(...) 評(píng)論(...) 編輯 收藏

轉(zhuǎn)載于:https://www.cnblogs.com/lonelyxmas/p/3567534.html

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