how to get eager loading in a many to many relationships?

问题

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.

回答1:

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) }

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

回答2:

I think you need this:

var q = myContext.EntityC.Include("EntityA").Include("EntityB").ToList();

If you want Bs of an A:

var aId; // = something...;

var bs = from c in q
                     where c.EntityAId == aId
                     select c.EntityBId;

And simply vice versa if you need As of a B:

var bId; // = something...;

var eas = from c in q
                     where c.EntityBId == bId
                     select c.EntityAId;

来源：https://stackoverflow.com/questions/18031702/how-to-get-eager-loading-in-a-many-to-many-relationships

标签

c#-4.0

entity-framework-5

eager-loading