如何在多对多的关系中得到急切的加载？ [英] how to get eager loading in a many to many relationships?

问题描述

我有一个有四个表的数据库。 TableA和TableB是主表，TableC是多对多关系的表。

• TableA（IDTableA，Name ... ）


• TableB（IDTableB，Name ...）


• TableC（IDTableA，IDTableB）

这创建了三个实体，EntityA有一个Entity C的ICollection，实体C有一个EntitiesB的集合，所以当我尝试获取相关的实体时，我这样做：

  myContext.EntityA.Include（a => a.EntityB.Select（b => b.EntityC））; 
  

但是这个throw和异常说明集合为null。

所以我想知道是否可以做一个急切的加载，当有一个表，多对多的关系。

谢谢。

解决方案

在Entity Framework中有许多关联，你可以在两种实现之间进行选择：

• 连接表（C）是概念模型（类模型）的一部分，关联是 A-C-B （1-n-1）。


• 联结表不是概念模型的一部分，但Entity Framework透明地使用它来维持关联 A-B （n-m）。 A具有B的集合，B具有As的集合。这只有在表C只包含两个FK列到A和B时才可能。

/ p>

你（显然）选择了第一个选项，所以你总是必须通过 C 查询其他的

  from a in context.As 
 select new {a，Bs = a.Cs.Select（c => ; cB）} 
  

或

<$ p $ （a => a1.Cs.Select（c => cB））

I have a database with four tables. TableA and TableB are the main tables and the TableC is the table of the many to many relationships.

• TableA(IDTableA, Name...)
• TableB(IDTableB, Name...)
• TableC(IDTableA, IDTableB)

This create three entities, The EntityA has an ICollection of Entity C and Entity C has a Collection of EntitiesB, so when I try to get the related entities I do this:

myContext.EntityA.Include(a=>a.EntityB.Select(b=>b.EntityC));

But this throw and exception that says that the collection is null.

So I would like to know if it is possible to do an eager loading when there are a table for the many to many relationship.

Thanks.

解决方案

With many to many association in Entity Framework you can choose between two implementations:

• The junction table (C) is part of the conceptual model (class model) and the associations are A—C—B (1—n—1). A can't have a collection of Bs.
• The junction table is not part of the conceptual model, but Entity Framework uses it transparently to sustain the association A—B (n—m). A has a collection of Bs and B has a collection of As. This is only possible when table C only contains the two FK columns to A and B.

So you can't have both.

You (apparently) chose the first option, so you will always have to query the other entites through C, like

from a in context.As
select new { a, Bs = a.Cs.Select(c => c.B) }

or

from a in As.Include(a1 => a1.Cs.Select(c => c.B))

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