實體框架中的一對多關係導致異常

Question

爲了向自我介紹實體框架，我創建了一個可與之協同工作的控制檯應用程序。

有兩個實體：A類和B類。它們之間有一對多的關係。一個ClassA實例可以有多個ClassB實例。 ClassB的一個實例有0或1個ClassA實例。

的類的代碼是：

public class ClassA { 
    public virtual int Id {get; set;} 
    public virtual string Name {get; set;} 
    public virtual ICollection<ClassB> ClassBs {get; set;} 
} 

public class ClassB { 
    public virtual int Id {get; set;} 
    public virtual string Name {get; set;} 
    public virtual ClassA ClassA {get; set;} 
} 

我的數據庫上下文和初始化代碼是：

public class Context : DbContext { 
    public DbSet<ClassA> ClassAs {get; set;} 
    public DbSet<ClassB> ClassBs {get; set;} 

    protected override void OnModelCreating(DbModelBuilder modelBuilder) { 
     modelBuilder.Entity<ClassA>() 
      .HasMany(classA => classA.ClassBs) 
      .WithRequired(classB => classB.ClassA); 
     base.OnModelCreating(modelBuilder); 
    } 
} 

public class Initializer : DropCreateDatabaseAlways<Context> { 
    protected override void Seed(Context context) { 
     base.Seed(context); 
     for (int i = 1; i <= 3; i++) { 
      var classA = new ClassA{ 
       Name = "A-" + i, 
       ClassBs = new LinkedList<ClassB>() 
      }; 
      for (int j = 1; j <= 3; j++) { 
       var classB = new ClassB{ 
        Name = "B-" + i + "-" + j, 
        ClassA = classA 
       }; 
       classA.ClassBs.Add(classB); 
       context.ClassBs.Add(classB); 
      } 
      context.ClassAs.Add(classA); 
     } 
     context.SaveChanges(); 
    } 
} 

我的主要方法是：

static void Main(string[] args) { 
     Database.SetInitializer(new Initializer()); 

     Context db = new Context(); 

     foreach (var classA in db.ClassAs) { 
      Console.WriteLine(classA.Name); 
      foreach (var classB in classA.ClassBs) 
       Console.WriteLine("\t" + classB.Name); 
     } 

     Console.Write("\nFIN"); 
     Console.ReadKey(); 
    } 

當第二Main方法中的foreach循環啓動以下異常拋出：

System.Data.EntityCommandExecutionException was unhandled 
    Message=An error occurred while executing the command definition. See the inner exception for details. 
    Source=System.Data.Entity 
    InnerException: System.InvalidOperationException 
     Message=There is already an open DataReader associated with this Command which must be closed first. 
     Source=System.Data 
     StackTrace: 
      at System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand command) 
      at System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String method, SqlCommand command) 
      at System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async) 
      at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result) 
      at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method) 
      at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method) 
      at System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior) 
      at System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior) 
      at System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior) 

看起來ClassA的ClassBs屬性沒有正確加載。但是我在上下文中指定了ClassA有很多ClassB。

數據庫連接看起來很好，因爲ClassA的第一個實例顯示在控制檯上。我也可以訪問Visual Studio中的數據庫並查看初始化程序中生成的數據。

我使用training videos of ASP.NET MVC開始使用此控制檯應用程序。

你知道發生了什麼問題嗎？

Answer 1

您需要在連接字符串中將multipleactiveresultsets設置爲true。

MultipleActiveResultSets = true;

Answer 2

該問題可能與模型的構建方式無關 - 更有可能與連接上是否啓用MultipleActiveResultSets啓用。請參閱Entity Framework: There is already an open DataReader associated with this Command