数据库

假设现在我们在SQL Server数据库中有下面两张表：

Person表，代表的是一个人：

  1. CREATE TABLE [dbo].[Person](
  2.     [ID] [int] IDENTITY(1,1) NOT NULL,
  3.     [PersonCode] [nvarchar](20) NULL,
  4.     [Name] [nvarchar](50) NULL,
  5.     [Age] [int] NULL,
  6.  CONSTRAINT [PK_Person] PRIMARY KEY CLUSTERED
  7. (
  8.     [ID] ASC
  9. )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
  10.  CONSTRAINT [IX_Person] UNIQUE NONCLUSTERED
  11. (
  12.     [PersonCode] ASC
  13. )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
  14. ) ON [PRIMARY]
  15. GO

其主键是ID，而且主键是自增列。Person表还有个PersonCode列是唯一键，然后Name和Age列用来描述一个人的名字和年龄。

Book表，代表的是一本书：

  1. CREATE TABLE [dbo].[Book](
  2.     [ID] [int] IDENTITY(1,1) NOT NULL,
  3.     [BookCode] [nvarchar](20) NULL,
  4.     [PersonCode] [nvarchar](20) NULL,
  5.     [BookName] [nvarchar](50) NULL,
  6.     [ISBN] [nvarchar](20) NULL,
  7.  CONSTRAINT [PK_Book] PRIMARY KEY CLUSTERED
  8. (
  9.     [ID] ASC
  10. )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
  11.  CONSTRAINT [IX_Book] UNIQUE NONCLUSTERED
  12. (
  13.     [BookCode] ASC
  14. )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
  15. ) ON [PRIMARY]
  16. GO

其主键是ID，而且主键也是自增列。Book表的BookCode列是唯一键，Book表的PersonCode列引用Person表的PersonCode列值，所以Book表的PersonCode列实际上是外键，但是我们并没有在数据库中设置两张表之间的外键关系，我们将稍后在EF Core中的实体之间设置外键关系，来演示就算在数据库中没有设置外键，EF Core也可以设置实体之间的外键关系。 所以Person表和Book表实际上是一对多关系，通过两张表的PersonCode列，一个Person对应多个Book，表示一个人可以拥有多本书。Book表还有BookName列和ISBN列，分别用来记录一本书的书名和ISBN号码。

实体

新建一个.NET Core控制台项目，现在我们在EF Core中建立Person表和Book表的实体：

Person实体，对应数据库的Person表，其属性Book是一个ICollection<Book>类型的Book实体集合，表示一个Person实体包含多个Book实体：

  1. public partial class Person
  2. {
  3.     public int Id { get; set; }
  4.     public string PersonCode { get; set; }
  5.     public string Name { get; set; }
  6.     public int? Age { get; set; }
  7.  
  8.     //通过Person实体的Book属性，可以找到多个Book实体，说明Person表是一对多关系中的主表
  9.     public virtual ICollection<Book> Book { get; set; }
  10. }

Book实体，对应数据库的Book表，其属性Person是一个Person实体，表示一个Book实体只能找到一个Person实体：

  1. public partial class Book
  2. {
  3.     public int Id { get; set; }
  4.     public string BookCode { get; set; }
  5.     public string PersonCode { get; set; }
  6.     public string BookName { get; set; }
  7.     public string Isbn { get; set; }
  8.  
  9.     //通过Book实体的Person属性，可以找到一个Person实体，说明Book表是一对多关系中的从表
  10.     public virtual Person Person { get; set; }
  11. }

然后是继承DbContext的TestDBContext类，其中最重要的地方是OnModelCreating方法中设置Person实体和Book实体一对多关系的Fluent API，每一行都写明了注释：

  1. public partial class TestDBContext : DbContext
  2. {
  3.     public TestDBContext()
  4.     {
  5.     }
  6.  
  7.     public TestDBContext(DbContextOptions<TestDBContext> options)
  8.         : base(options)
  9.     {
  10.     }
  11.  
  12.     public virtual DbSet<Book> Book { get; set; }
  13.     public virtual DbSet<Person> Person { get; set; }
  14.  
  15.     protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
  16.     {
  17.         if (!optionsBuilder.IsConfigured)
  18.         {
  19.             optionsBuilder.UseSqlServer("Server=localhost;User Id=sa;Password=Dtt!123456;Database=TestDB");
  20.         }
  21.     }
  22.  
  23.     protected override void OnModelCreating(ModelBuilder modelBuilder)
  24.     {
  25.         modelBuilder.Entity<Book>(entity =>
  26.         {
  27.             entity.HasKey(e => e.BookCode);//设置Book实体的BookCode属性为EF Core实体的Key属性
  28.  
  29.             entity.HasIndex(e => e.BookCode)
  30.                 .HasName("IX_Book")
  31.                 .IsUnique();
  32.  
  33.             entity.Property(e => e.Id).ValueGeneratedOnAdd();//设置Book实体的Id属性为插入数据到数据库Book表时自动生成，因为Book表的ID列为自增列
  34.             entity.Property(e => e.Id).HasColumnName("ID");
  35.  
  36.             entity.Property(e => e.BookCode).HasMaxLength();
  37.  
  38.             entity.Property(e => e.BookName).HasMaxLength();
  39.  
  40.             entity.Property(e => e.Isbn)
  41.                 .HasColumnName("ISBN")
  42.                 .HasMaxLength();
  43.  
  44.             entity.Property(e => e.PersonCode).HasMaxLength();
  45.         });
  46.  
  47.         modelBuilder.Entity<Person>(entity =>
  48.         {
  49.             entity.HasKey(e => e.PersonCode);//设置Person实体的PersonCode属性为EF Core实体的Key属性
  50.  
  51.             entity.HasIndex(e => e.PersonCode)
  52.                 .HasName("IX_Person")
  53.                 .IsUnique();
  54.  
  55.             entity.Property(e => e.Id).ValueGeneratedOnAdd();//设置Person实体的Id属性为插入数据到数据库Person表时自动生成，因为Person表的ID列为自增列
  56.             entity.Property(e => e.Id).HasColumnName("ID");
  57.  
  58.             entity.Property(e => e.Name).HasMaxLength();
  59.  
  60.             entity.Property(e => e.PersonCode).HasMaxLength();
  61.  
  62.             //设置Person实体和Book实体之间的一对多关系，尽管我们并没有在数据库中建立Person表和Book表之间的一对多外键关系，但是我们可以用EF Core的Fluent API在实体层面设置外键关系
  63.             entity.HasMany(p => p.Book)//设置Person实体通过属性Book可以找到多个Book实体，表示Person表是一对多关系中的主表
  64.             .WithOne(b => b.Person)//设置Book实体通过属性Person可以找到一个Person实体，表示Book表是一对多关系中的从表
  65.             .HasPrincipalKey(p => p.PersonCode)//设置Person表的PersonCode列为一对多关系中的主表键
  66.             .HasForeignKey(b => b.PersonCode)//设置Book表的PersonCode列为一对多关系中的从表外键
  67.             .OnDelete(DeleteBehavior.ClientSetNull);//设置一对多关系的级联删除效果为DeleteBehavior.ClientSetNull
  68.  
  69.         });
  70.     }
  71. }

示例代码

现在我们来设想下面一个场景：

假设数据库中的Person表有一行数据如下：

数据库中的Book表有三行数据如下：

可以看到Book表三行数据的PersonCode列都为NULL，那么我们怎么在EF Core中更改Book表三行数据的PersonCode列为Person表的PersonCode列值呢？也就是说将Book表三行数据的PersonCode列都改为Person表的值P001，从而表示James这个人拥有三本书。

本例的示例代码都写在了.NET Core控制台项目的Program类中，这里先将代码全部贴出来：

  1. class Program
  2. {
  3.     /// <summary>
  4.     /// 初始化Person表和Book表的数据，没有设置Book表的外键列PersonCode的值
  5.     /// </summary>
  6.     static void InitData()
  7.     {
  8.         //初始化数据库数据
  9.         using (var dbContext = new TestDBContext())
  10.         {
  11.             var james = new Person() { PersonCode = "P001", Name = "James", Age =  };
  12.  
  13.             dbContext.Person.Add(james);
  14.  
  15.             var chineseBook = new Book() { BookCode = "B001", Isbn = "", BookName = "Chinese" };//没有设置Book表中外键列PersonCode的值
  16.             var japaneseBook = new Book() { BookCode = "B002", Isbn = "", BookName = "Japanese" };//没有设置Book表中外键列PersonCode的值
  17.             var englishBook = new Book() { BookCode = "B003", Isbn = "", BookName = "English" };//没有设置Book表中外键列PersonCode的值
  18.  
  19.             //插入三条数据到Book表
  20.             dbContext.Book.Add(chineseBook);
  21.             dbContext.Book.Add(japaneseBook);
  22.             dbContext.Book.Add(englishBook);
  23.  
  24.             dbContext.SaveChanges();
  25.         }
  26.     }
  27.  
  28.     /// <summary>
  29.     /// 删除Person表和Book表的所有数据
  30.     /// </summary>
  31.     static void DeleteAllData()
  32.     {
  33.         using (var dbContext = new TestDBContext())
  34.         {
  35.             dbContext.Database.ExecuteSqlCommand("DELETE FROM [dbo].[Book]");
  36.             dbContext.Database.ExecuteSqlCommand("DELETE FROM [dbo].[Person]");
  37.         }
  38.     }
  39.  
  40.     /// <summary>
  41.     /// 不正确地设置Person表和Book表的关联关系，这种方法会让EF Core错误地生成INSERT语句，而不是UPDATE语句
  42.     /// </summary>
  43.     static void SetRelationshipIncorrectly()
  44.     {
  45.         using (var dbContext = new TestDBContext())
  46.         {
  47.             var james = dbContext.Person.First(e => e.Name == "James");//首先通过DbContext从数据库中查询出要建立关联关系的Person表实体
  48.  
  49.             var chineseBook = new Book() { BookCode = "B001" };//只构造Book实体的Key属性即可，根据BookCode值"B001"来构造Chinese Book
  50.             var japaneseBook = new Book() { BookCode = "B002" };//只构造Book实体的Key属性即可，根据BookCode值"B002"来构造Japanese Book
  51.             var englishBook = new Book() { BookCode = "B003" };//只构造Book实体的Key属性即可，根据BookCode值"B003"来构造English Book
  52.  
  53.             Console.WriteLine($"Before adding, chineseBook entity state is :{dbContext.Entry(chineseBook).State.ToString()}");//可以看到由于此时Book实体chineseBook没有被DbContext跟踪，所以状态是Detached
  54.             Console.WriteLine($"Before adding, japaneseBook entity state is :{dbContext.Entry(japaneseBook).State.ToString()}");//可以看到由于此时Book实体japaneseBook没有被DbContext跟踪，所以状态是Detached
  55.             Console.WriteLine($"Before adding, englishBook entity state is :{dbContext.Entry(englishBook).State.ToString()}");//可以看到由于此时Book实体englishBook没有被DbContext跟踪，所以状态是Detached
  56.  
  57.             Console.WriteLine();
  58.  
  59.             james.Book = new List<Book>();//由于我们在上面调用dbContext.Person.First(e => e.Name == "James")时，没有用EF Core中Eager Loading的Include方法来加载Book实体集合，所以这里要用List类来构造一个Book实体集合，否则james.Book为null
  60.  
  61.             james.Book.Add(chineseBook);//添加chineseBook到Person类的Book实体集合
  62.             Console.WriteLine("chineseBook was added into Person.Book collection");
  63.  
  64.             james.Book.Add(japaneseBook);//添加japaneseBook到Person类的Book实体集合
  65.             Console.WriteLine("japaneseBook was added into Person.Book collection");
  66.  
  67.             james.Book.Add(englishBook);//添加englishBook到Person类的Book实体集合
  68.             Console.WriteLine("englishBook was added into Person.Book collection");
  69.  
  70.             Console.WriteLine();
  71.  
  72.             Console.WriteLine($"After querying DbContext.Entry(chineseBook), chineseBook entity state is :{dbContext.Entry(chineseBook).State.ToString()}");//调用DbContext.Entry()方法后，DbContext发现一个原本状态是Detached的Book实体chineseBook被加入到Person.Book集合中了，所以此时chineseBook的实体状态变为了Added
  73.             Console.WriteLine($"After querying DbContext.Entry(japaneseBook), japaneseBook entity state is :{dbContext.Entry(japaneseBook).State.ToString()}");//调用DbContext.Entry()方法后，DbContext发现一个原本状态是Detached的Book实体japaneseBook被加入到Person.Book集合中了，所以此时japaneseBook的实体状态变为了Added
  74.             Console.WriteLine($"After querying DbContext.Entry(englishBook), englishBook entity state is :{dbContext.Entry(englishBook).State.ToString()}");//调用DbContext.Entry()方法后，DbContext发现一个原本状态是Detached的Book实体englishBook被加入到Person.Book集合中了，所以此时englishBook的实体状态变为了Added
  75.  
  76.             dbContext.SaveChanges();//由于此时chineseBook、japaneseBook和englishBook的EntityState都是Added，所以此时DbContext.SaveChanges方法调用后，EF Core生成的是INSERT语句，将chineseBook、japaneseBook和englishBook插入数据库表Book，导致插入了重复值到唯一键列BookCode，所以数据库报错
  77.         }
  78.     }
  79.  
  80.     /// <summary>
  81.     /// 正确地设置Person表和Book表的关联关系，这种方法会让EF Core正确地生成UPDATE语句，在数据库中设置Book表的PersonCode列数据
  82.     /// </summary>
  83.     static void SetRelationshipCorrectly()
  84.     {
  85.         using (var dbContext = new TestDBContext())
  86.         {
  87.             var james = dbContext.Person.First(e => e.Name == "James");//首先通过DbContext从数据库中查询出要建立关联关系的Person表实体
  88.  
  89.             var chineseBook = new Book() { BookCode = "B001" };//只构造Book实体的Key属性即可，根据BookCode值"B001"来构造Chinese Book
  90.             var japaneseBook = new Book() { BookCode = "B002" };//只构造Book实体的Key属性即可，根据BookCode值"B002"来构造Japanese Book
  91.             var englishBook = new Book() { BookCode = "B003" };//只构造Book实体的Key属性即可，根据BookCode值"B003"来构造English Book
  92.  
  93.             dbContext.Attach(chineseBook);//将chineseBook关联到DbContext，开始跟踪
  94.             dbContext.Attach(japaneseBook);//将japaneseBook关联到DbContext，开始跟踪
  95.             dbContext.Attach(englishBook);//将englishBook关联到DbContext，开始跟踪
  96.  
  97.             Console.WriteLine($"After querying DbContext.Entry(chineseBook), chineseBook entity state is :{dbContext.Entry(chineseBook).State.ToString()}");//由于上面chineseBook被Attach到DbContext开始跟踪了，所以此时chineseBook的实体状态是Unchanged
  98.             Console.WriteLine($"After querying DbContext.Entry(japaneseBook), japaneseBook entity state is :{dbContext.Entry(japaneseBook).State.ToString()}");//由于上面japaneseBook被Attach到DbContext开始跟踪了，所以此时japaneseBook的实体状态是Unchanged
  99.             Console.WriteLine($"After querying DbContext.Entry(englishBook), englishBook entity state is :{dbContext.Entry(englishBook).State.ToString()}");//由于上面englishBook被Attach到DbContext开始跟踪了，所以此时englishBook的实体状态是Unchanged
  100.  
  101.             Console.WriteLine();
  102.  
  103.             james.Book = new List<Book>();//由于我们在上面调用dbContext.Person.First(e => e.Name == "James")时，没有用EF Core中Eager Loading的Include方法来加载Book实体集合，所以这里要用List类来构造一个Book实体集合，否则james.Book为null
  104.  
  105.             james.Book.Add(chineseBook);//添加chineseBook到Person类的Book实体集合
  106.             Console.WriteLine("chineseBook was added into Person.Book collection");
  107.  
  108.             james.Book.Add(japaneseBook);//添加japaneseBook到Person类的Book实体集合
  109.             Console.WriteLine("japaneseBook was added into Person.Book collection");
  110.  
  111.             james.Book.Add(englishBook);//添加englishBook到Person类的Book实体集合
  112.             Console.WriteLine("englishBook was added into Person.Book collection");
  113.  
  114.             Console.WriteLine();
  115.  
  116.             Console.WriteLine($"Berfore querying DbContext.Entry(chineseBook), chineseBook.PersonCode is :{chineseBook.PersonCode ?? "null"}");//此时由于我们还没有调用DbContext.Entry()方法，所以DbContext还无法察觉到chineseBook已经被添加到Person类的Book实体集合了，所以chineseBook.PersonCode为null
  117.             Console.WriteLine($"After querying DbContext.Entry(chineseBook), chineseBook entity state is :{dbContext.Entry(chineseBook).State.ToString()}");//调用DbContext.Entry()方法后，DbContext发现一个原本状态是Unchanged的Book实体chineseBook被加入到Person.Book集合中了，所以此时chineseBook的实体状态变为了Modified
  118.             Console.WriteLine($"After querying DbContext.Entry(chineseBook),  chineseBook.PersonCode is :{chineseBook.PersonCode}");//由于上面我们调用DbContext.Entry(chineseBook)使得DbContext得知了chineseBook被加入到Person.Book集合中了，所以DbContext还将Book实体的外键属性PersonCode也进行了赋值，为P001
  119.  
  120.             Console.WriteLine();
  121.  
  122.             Console.WriteLine($"Berfore querying DbContext.Entry(japaneseBook),japaneseBook.PersonCode is :{japaneseBook.PersonCode ?? "null"}");//很有意思的是我们上面在chineseBook上调用DbContext.Entry()方法后，japaneseBook的PersonCode属性也不为null了，变为了P001，说明调用一次DbContext.Entry()方法后，会引发DbContext重新检查所有被跟踪实体的状态
  123.             Console.WriteLine($"After querying DbContext.Entry(japaneseBook), japaneseBook entity state is :{dbContext.Entry(japaneseBook).State.ToString()}");//在上面为chineseBook调用DbContext.Entry()方法时，DbContext同时发现了原本状态是Unchanged的Book实体japaneseBook，也被加入到了Person.Book集合中，所以japaneseBook的实体状态也变为了Modified
  124.             Console.WriteLine($"After querying DbContext.Entry(japaneseBook),  japaneseBook.PersonCode is :{japaneseBook.PersonCode}");//在上面为chineseBook调用DbContext.Entry()方法时,DbContext得知了japaneseBook也被加入到了Person.Book集合中，所以DbContext将japaneseBook的PersonCode属性也赋值为P001了
  125.  
  126.             Console.WriteLine();
  127.  
  128.             Console.WriteLine($"Berfore querying DbContext.Entry(englishBook),englishBook.PersonCode is :{englishBook.PersonCode ?? "null"}");//在上面为chineseBook调用DbContext.Entry()方法时,DbContext得知了englishBook也被加入到了Person.Book集合中，所以DbContext将englishBook的PersonCode属性也赋值为P001了
  129.             Console.WriteLine($"After querying DbContext.Entry(englishBook), englishBook entity state is :{dbContext.Entry(englishBook).State.ToString()}");//在上面为chineseBook调用DbContext.Entry()方法时，DbContext同时发现了原本状态是Unchanged的Book实体englishBook，也被加入到了Person.Book集合中，所以englishBook的实体状态也变为了Modified
  130.             Console.WriteLine($"After querying DbContext.Entry(englishBook),  englishBook.PersonCode is :{englishBook.PersonCode}");//在上面为chineseBook调用DbContext.Entry()方法时,DbContext得知了englishBook也被加入到了Person.Book集合中，所以DbContext将englishBook的PersonCode属性也赋值为P001了
  131.  
  132.             dbContext.SaveChanges();//由于此时chineseBook、japaneseBook和englishBook的EntityState都是Modified，所以此时DbContext.SaveChanges方法调用后，EF Core生成的是UPDATE语句，通过更新数据库Book表的PersonCode列，将Chinese、Japanese和English三行Book数据同Person表的数据成功关联了起来
  133.         }
  134.     }
  135.  
  136.     static void Main(string[] args)
  137.     {
  138.         DeleteAllData();//调用DeleteAllData方法删除Person表和Book表的所有数据，防止有脏数据
  139.         InitData();//初始化Person表和Book表的数据
  140.  
  141.         SetRelationshipCorrectly();//正确地的设置Person表和Book表的关联关系
  142.         SetRelationshipIncorrectly();//不正确地的设置Person表和Book表的关联关系，该方法会抛出异常错误
  143.  
  144.         Console.WriteLine("Press any key to quit...");
  145.         Console.ReadKey();
  146.     }
  147. }

示例代码中的DeleteAllData方法，是清表语句，用来删除Person表和Book表的所有数据，防止有脏数据。

InitData方法用来初始化Person表和Book表的数据，Person表插入了一行数据，Book表插入了三行数据且PersonCode列都为NULL，调用InitData方法后数据库Person表和Book表的数据就和上面示例代码前的两个截图相同了。

测试SetRelationshipIncorrectly方法

先将示例代码的Main方法改为如下：

  1. static void Main(string[] args)
  2. {
  3.     DeleteAllData();//调用DeleteAllData方法删除Person表和Book表的所有数据，防止有脏数据
  4.     InitData();//初始化Person表和Book表的数据
  5.  
  6.     //SetRelationshipCorrectly();//正确地的设置Person表和Book表的关联关系
  7.     SetRelationshipIncorrectly();//不正确地的设置Person表和Book表的关联关系，该方法会抛出异常错误
  8.  
  9.     Console.WriteLine("Press any key to quit...");
  10.     Console.ReadKey();
  11. }

SetRelationshipIncorrectly方法用来演示怎么错误地设置Person表和Book表的关联关系，可以看到由于我们在其中新建的三个Book实体

  1. var chineseBook = new Book() { BookCode = "B001" };//只构造Book实体的Key属性即可，根据BookCode值"B001"来构造Chinese Book
  2. var japaneseBook = new Book() { BookCode = "B002" };//只构造Book实体的Key属性即可，根据BookCode值"B002"来构造Japanese Book
  3. var englishBook = new Book() { BookCode = "B003" };//只构造Book实体的Key属性即可，根据BookCode值"B003"来构造English Book

最终在调用DbContext.SaveChanges方法时其实体状态都是Added，所以调用DbContext.SaveChanges方法时，EF Core在数据库中生成的是INSERT语句，尝试将这三个实体数据插入数据库Book表，由于调用InitData方法后，数据库Book表中已经有相同PersonCode列值的数据了，Book表的PersonCode列又是唯一键，所以DbContext.SaveChanges方法抛出异常。

我们可以从EF Core的后台日志中，查看到调用DbContext.SaveChanges方法时生成的是INSERT语句：

  1. =============================== EF Core log started ===============================
  2. SaveChanges starting for 'TestDBContext'.
  3. =============================== EF Core log finished ===============================
  4. =============================== EF Core log started ===============================
  5. DetectChanges starting for 'TestDBContext'.
  6. =============================== EF Core log finished ===============================
  7. =============================== EF Core log started ===============================
  8. DetectChanges completed for 'TestDBContext'.
  9. =============================== EF Core log finished ===============================
  10. =============================== EF Core log started ===============================
  11. Opening connection to database 'TestDB' on server 'localhost'.
  12. =============================== EF Core log finished ===============================
  13. =============================== EF Core log started ===============================
  14. Opened connection to database 'TestDB' on server 'localhost'.
  15. =============================== EF Core log finished ===============================
  16. =============================== EF Core log started ===============================
  17. Beginning transaction with isolation level 'ReadCommitted'.
  18. =============================== EF Core log finished ===============================
  19. =============================== EF Core log started ===============================
  20. Executing update commands individually as the number of batchable commands (3) is smaller than the minimum batch size (4).
  21. =============================== EF Core log finished ===============================
  22. =============================== EF Core log started ===============================
  23. Executing DbCommand [Parameters=[@p0='?' (Size = 20), @p1='?' (Size = 50), @p2='?' (Size = 20), @p3='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  24. SET NOCOUNT ON;
  25. INSERT INTO [Book] ([BookCode], [BookName], [ISBN], [PersonCode])
  26. VALUES (@p0, @p1, @p2, @p3);
  27. SELECT [ID]
  28. FROM [Book]
  29. WHERE @@ROWCOUNT = 1 AND [BookCode] = @p0;
  30. =============================== EF Core log finished ===============================
  31. =============================== EF Core log started ===============================
  32. Failed executing DbCommand (8ms) [Parameters=[@p0='?' (Size = 20), @p1='?' (Size = 50), @p2='?' (Size = 20), @p3='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  33. SET NOCOUNT ON;
  34. INSERT INTO [Book] ([BookCode], [BookName], [ISBN], [PersonCode])
  35. VALUES (@p0, @p1, @p2, @p3);
  36. SELECT [ID]
  37. FROM [Book]
  38. WHERE @@ROWCOUNT = 1 AND [BookCode] = @p0;
  39. =============================== EF Core log finished ===============================
  40. =============================== EF Core log started ===============================
  41. Disposing transaction.
  42. =============================== EF Core log finished ===============================
  43. =============================== EF Core log started ===============================
  44. Closing connection to database 'TestDB' on server 'localhost'.
  45. =============================== EF Core log finished ===============================
  46. =============================== EF Core log started ===============================
  47. Closed connection to database 'TestDB' on server 'localhost'.
  48. =============================== EF Core log finished ===============================

在SetRelationshipIncorrectly方法中我们还输出了chineseBook、japaneseBook和englishBook三个Book实体的EntityState，可以看到将chineseBook、japaneseBook和englishBook三个Book实体添加到Person类的Book实体集合后，EntityState发生了相应的变化。

测试SetRelationshipCorrectly方法

先将示例代码的Main方法改为如下：

  1. static void Main(string[] args)
  2. {
  3.     DeleteAllData();//调用DeleteAllData方法删除Person表和Book表的所有数据，防止有脏数据
  4.     InitData();//初始化Person表和Book表的数据
  5.  
  6.     SetRelationshipCorrectly();//正确地的设置Person表和Book表的关联关系
  7.     //SetRelationshipIncorrectly();//不正确地的设置Person表和Book表的关联关系，该方法会抛出异常错误
  8.  
  9.     Console.WriteLine("Press any key to quit...");
  10.     Console.ReadKey();
  11. }

SetRelationshipCorrectly方法用来演示如何正确地设置Person表和Book表的关联关系，首先我们将

  1. var chineseBook = new Book() { BookCode = "B001" };//只构造Book实体的Key属性即可，根据BookCode值"B001"来构造Chinese Book
  2. var japaneseBook = new Book() { BookCode = "B002" };//只构造Book实体的Key属性即可，根据BookCode值"B002"来构造Japanese Book
  3. var englishBook = new Book() { BookCode = "B003" };//只构造Book实体的Key属性即可，根据BookCode值"B003"来构造English Book

这三个新建的Book实体Attach到了DbContext，如下所示：

  1. dbContext.Attach(chineseBook);//将chineseBook关联到DbContext，开始跟踪
  2. dbContext.Attach(japaneseBook);//将japaneseBook关联到DbContext，开始跟踪
  3. dbContext.Attach(englishBook);//将englishBook关联到DbContext，开始跟踪

所以这次最终在调用DbContext.SaveChanges方法时其实体状态都是Modified，那么调用DbContext.SaveChanges方法时，EF Core在数据库中生成的是UPDATE语句，正确地将chineseBook、japaneseBook和englishBook这三个Book实体的PersonCode列值更新到了数据库Book表中，我们可以从EF Core的后台日志中，查看到调用DbContext.SaveChanges方法时生成的是UPDATE语句：

  1. =============================== EF Core log started ===============================
  2. SaveChanges starting for 'TestDBContext'.
  3. =============================== EF Core log finished ===============================
  4. =============================== EF Core log started ===============================
  5. DetectChanges starting for 'TestDBContext'.
  6. =============================== EF Core log finished ===============================
  7. =============================== EF Core log started ===============================
  8. DetectChanges completed for 'TestDBContext'.
  9. =============================== EF Core log finished ===============================
  10. =============================== EF Core log started ===============================
  11. Opening connection to database 'TestDB' on server 'localhost'.
  12. =============================== EF Core log finished ===============================
  13. =============================== EF Core log started ===============================
  14. Opened connection to database 'TestDB' on server 'localhost'.
  15. =============================== EF Core log finished ===============================
  16. =============================== EF Core log started ===============================
  17. Beginning transaction with isolation level 'ReadCommitted'.
  18. =============================== EF Core log finished ===============================
  19. =============================== EF Core log started ===============================
  20. Executing update commands individually as the number of batchable commands (3) is smaller than the minimum batch size (4).
  21. =============================== EF Core log finished ===============================
  22. =============================== EF Core log started ===============================
  23. Executing DbCommand [Parameters=[@p1='?' (Size = 20), @p0='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  24. SET NOCOUNT ON;
  25. UPDATE [Book] SET [PersonCode] = @p0
  26. WHERE [BookCode] = @p1;
  27. SELECT @@ROWCOUNT;
  28. =============================== EF Core log finished ===============================
  29. =============================== EF Core log started ===============================
  30. Executed DbCommand (4ms) [Parameters=[@p1='?' (Size = 20), @p0='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  31. SET NOCOUNT ON;
  32. UPDATE [Book] SET [PersonCode] = @p0
  33. WHERE [BookCode] = @p1;
  34. SELECT @@ROWCOUNT;
  35. =============================== EF Core log finished ===============================
  36. =============================== EF Core log started ===============================
  37. A data reader was disposed.
  38. =============================== EF Core log finished ===============================
  39. =============================== EF Core log started ===============================
  40. Executing DbCommand [Parameters=[@p1='?' (Size = 20), @p0='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  41. SET NOCOUNT ON;
  42. UPDATE [Book] SET [PersonCode] = @p0
  43. WHERE [BookCode] = @p1;
  44. SELECT @@ROWCOUNT;
  45. =============================== EF Core log finished ===============================
  46. =============================== EF Core log started ===============================
  47. Executed DbCommand (4ms) [Parameters=[@p1='?' (Size = 20), @p0='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  48. SET NOCOUNT ON;
  49. UPDATE [Book] SET [PersonCode] = @p0
  50. WHERE [BookCode] = @p1;
  51. SELECT @@ROWCOUNT;
  52. =============================== EF Core log finished ===============================
  53. =============================== EF Core log started ===============================
  54. A data reader was disposed.
  55. =============================== EF Core log finished ===============================
  56. =============================== EF Core log started ===============================
  57. Executing DbCommand [Parameters=[@p1='?' (Size = 20), @p0='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  58. SET NOCOUNT ON;
  59. UPDATE [Book] SET [PersonCode] = @p0
  60. WHERE [BookCode] = @p1;
  61. SELECT @@ROWCOUNT;
  62. =============================== EF Core log finished ===============================
  63. =============================== EF Core log started ===============================
  64. Executed DbCommand (4ms) [Parameters=[@p1='?' (Size = 20), @p0='?' (Size = 20)], CommandType='Text', CommandTimeout='30']
  65. SET NOCOUNT ON;
  66. UPDATE [Book] SET [PersonCode] = @p0
  67. WHERE [BookCode] = @p1;
  68. SELECT @@ROWCOUNT;
  69. =============================== EF Core log finished ===============================
  70. =============================== EF Core log started ===============================
  71. A data reader was disposed.
  72. =============================== EF Core log finished ===============================
  73. =============================== EF Core log started ===============================
  74. Committing transaction.
  75. =============================== EF Core log finished ===============================
  76. =============================== EF Core log started ===============================
  77. Closing connection to database 'TestDB' on server 'localhost'.
  78. =============================== EF Core log finished ===============================
  79. =============================== EF Core log started ===============================
  80. Closed connection to database 'TestDB' on server 'localhost'.
  81. =============================== EF Core log finished ===============================
  82. =============================== EF Core log started ===============================
  83. Disposing transaction.
  84. =============================== EF Core log finished ===============================
  85. =============================== EF Core log started ===============================
  86. An 'Book' entity tracked by 'TestDBContext' changed from 'Modified' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values.
  87. =============================== EF Core log finished ===============================
  88. =============================== EF Core log started ===============================
  89. An 'Book' entity tracked by 'TestDBContext' changed from 'Modified' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values.
  90. =============================== EF Core log finished ===============================
  91. =============================== EF Core log started ===============================
  92. An 'Book' entity tracked by 'TestDBContext' changed from 'Modified' to 'Unchanged'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see key values.
  93. =============================== EF Core log finished ===============================
  94. =============================== EF Core log started ===============================
  95. SaveChanges completed for 'TestDBContext' with 3 entities written to the database.
  96. =============================== EF Core log finished ===============================

最终数据库中Person表和Book表的数据如下所示：

Person表

Book表

在SetRelationshipCorrectly方法中我们也输出了chineseBook、japaneseBook和englishBook三个Book实体的EntityState，可以看到这三个Book实体的EntityState发生的变化

DbContext何时会检查被跟踪实体的状态？

从目前得知的情况，调用DbContext.Entry方法和DbContext.SaveChanges方法时，都会触发DbContext对所有被跟踪实体的重新检查，从而更新被跟踪实体的EntityState和相关实体属性。在EF Core中应该还有其它方法也会触发DbContext重新检查被跟踪实体的状态，后面发现后再做补充。

更正

本文中示例代码中的SetRelationshipIncorrectly和SetRelationshipCorrectly两个方法，有一段描述有点问题，如下：

  1. james.Book = new List<Book>();//由于我们在上面调用dbContext.Person.First(e => e.Name == "James")时，没有用EF Core中Eager Loading的Include方法来加载Book实体集合，所以这里要用List类来构造一个Book实体集合，否则james.Book为null

其实我发现在EF Core中就算没开启Lazy Loading和没使用Eager Loading的Include方法时，使用DbContext做查询后（例如上面代码中的james就来自查询dbContext.Person.First(e => e.Name == "James")），也会给集合导航属性赋值为System.Collections.Generic.HashSet<T>，只不过其长度(Count)为0。

所以上面这段代码给james.Book赋值其实是没有必要的，因为james.Book本来就不为null，后面直接调用james.Book的Add方法即可。只不过貌似给james.Book赋值为new List<Book>()也没有什么问题，只是没有必要而已。

源代码下载

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