序列不包含继承元素错误 [英] Sequence contains no elements error with inheritance

问题描述

  public abstract class MyBaseClass 
 {
 public int Id {get;组; } 
 
 public int OwnerId {get;组; } 
 public virtual Org Owner {get;组; 
 
} 
 
 public abstract class Party：MyBaseClass 
 {
 
} 
 
 public class Org： Party 
 {
 public string Name {get;组; } 
 public DateTime CreationDate {get;组; 
} 
  

我的数据上下文有这样的一行：

  public DbSet< Org>组织{get;组; } 
  

执行这样的查询时

  var orgs = db.Orgs; 
  

我收到此错误：

多重性在关系
'Org_Owner'中的角色'Org_Owner_Source'中无效。因为从属角色属性不是关键的
属性，所以依赖角色
的多重性的上限必须为'*'。

为了克服这个问题，我添加了以下Fluent API代码：

  modelBuilder.Entity< ; MyBaseClass>（）
 .HasRequired（e => e.Owner）
 .WithMany（）
 .HasForeignKey（e => e.OwnerId）
 .WillCascadeOnDelete假）; 
  

这解决了无效的多重性错误，但是我随后运行我的查询，它返回一个序列不包含元素错误。当我检查生成的SQL代码正在击中数据库时，我发现来自'子句的查询'引用了一个不存在的MyBaseClassClasses表而不是Orgs表的预期

运行迁移还会引发序列不包含元素错误。无论如何，我不希望所有从 MyBaseClass 继承的类被非规范化到一个MyBaseClassClasses表中。

问题：

1. 在这种情况下，流利的api代码是否正确？


2. 如果没有，什么应该是吗？


3. 如果是，那么我如何克服序列不包含元素错误？

** * ** 后续修改 * ** > * ** * ** * ** * ** * **

另一种我一直在尝试的方式是删除Fluid Mapping和以下类定义：

  public abstract class MyBaseClass 
 {
 public int Id {get;组; } 
 
 [ForeignKey（Owner）] 
 public int OwnerId {get;组; } 
 public virtual Org Owner {get;组; 
 
} 
 
 public abstract class Party：MyBaseClass 
 {
 
} 
 
 public class Org： Party 
 {
 public string Name {get;组; } 
 public DateTime CreationDate {get;组; } 
} 
  

此解决方案会抛出Multiplicity在角色Org_Owner_Source中无效关系Org_Owner，因为从属角色属性不是关键属性，因此从属角色的多重性的上限必须为*。错误。

我能够部分解决问题，因为我发现除了Org之外，从MyBaseClass继承的任何其他类都可以拥有一个延迟加载Owner属性，如果属性直接输入到小孩类中，并输入流畅的api条目。

例如：

  public class MyOtherClass：MyBaseClass 
 {
 public string Name {get;组; } 
 public string Whaterver {get;组; } 
 public virtual Org Owner {get;组; 
} 
  

使用流体API条目：

  modelBuilder.Entity< MyOtherClass>（）。HasRequired（x => x.Owner）.WithMany（）。HasForeignKey（x => x.OwnerId）。 WillCascadeOnDelete（真）; 
  

解决方案

这是不正确的流畅映射。通过调用 modelBuilder.Entity< MyBaseClass>（），您正在通知实体框架 MyBaseClass 是一个实体，应该以某种方式映射到数据库。而是使用您要映射的最低级别。

例如：

  modelBuilder.Entity< Org>（）。HasRequired（e => e.Owner）; 
  

I have a base class from which almost everything else inherits..

public abstract class MyBaseClass
{
   public int Id { get; set; }

   public int OwnerId{ get; set; }
   public virtual Org Owner{ get; set; }

}

public abstract class Party: MyBaseClass
{

}

public class Org: Party
{
   public string Name { get; set; }
   public DateTime CreationDate { get; set; }
}

My data context has a line like this:

public DbSet<Org> Orgs { get; set; }

When executing a query like this...

var orgs = db.Orgs;

I get this error:

Multiplicity is not valid in Role 'Org_Owner_Source' in relationship 'Org_Owner'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'.

In order to overcome this problem, I added the following Fluent API code:

    modelBuilder.Entity<MyBaseClass>()
                .HasRequired(e => e.Owner)
                .WithMany()
                .HasForeignKey(e => e.OwnerId)
                .WillCascadeOnDelete(false);

This resolves the Invalid Multiplicity error but when I subsequently run my query, it returns a "Sequence contains no elements error". When I inspect the generated SQL code that is hitting the database, I find that the query's 'from' clause is referencing a non-existent MyBaseClassClasses table instead of the Orgs table as expected.

Running a migration also throws the 'Sequence contains no elements' error. Anyway, I wouldn't want all the classes that inherit from MyBaseClass to be denormalized into a single MyBaseClassClasses table.

Questions:

1. Is the fluent api code correct in this situation?
2. If no, what should it be?
3. If yes, then how can I overcome the 'Sequence contains no elements' error?

***** Follow-up Edit ******************

The other way I have been trying it is with the Fluid Mapping removed and the following class definitions:

public abstract class MyBaseClass
{
   public int Id { get; set; }

   [ForeignKey("Owner")]
   public int OwnerId{ get; set; }
   public virtual Org Owner{ get; set; }

}

public abstract class Party: MyBaseClass
{

}

public class Org: Party
{
   public string Name { get; set; }
   public DateTime CreationDate { get; set; }
}

This solution throws the "Multiplicity is not valid in Role 'Org_Owner_Source' in relationship 'Org_Owner'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'." error.

I was able to partially work around the problem as I discovered that any other class, besides Org, that inherits from MyBaseClass can have a lazy loading Owner attribute if the attribute is entered directly in to child class and a fluid api entry is made.

For example:

public class MyOtherClass: MyBaseClass
{
   public string Name { get; set; }
   public string Whaterver { get; set; }
   public virtual Org Owner{ get; set; }
}

With the fluid API entry:

modelBuilder.Entity<MyOtherClass>().HasRequired(x => x.Owner).WithMany().HasForeignKey(x=>x.OwnerId).WillCascadeOnDelete(true);

解决方案

That is not the correct fluent mapping. By calling modelBuilder.Entity<MyBaseClass>(), you are telling Entity Framework that MyBaseClass is an entity and should somehow map to the database. Instead, use lowest class that you want mapped.

For example:

modelBuilder.Entity<Org>().HasRequired(e => e.Owner);

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