EF4 How to expose the join table in a many to many

问题:

Say I have the following tables:

Essence, EssenceSet, and Essence2EssenceSet where Essence2EssenceSet holds just the IDs of the 1st 2 tables to form the M:M relationship.

In EF since Essence2EssenceSet has no other fields it is not exposed in the model. I find this makes it difficult to insert records into this table when I already have the 2 IDs needed to create the record but don't necessarily have the Essence and EssenceSet records loaded (Just their IDs)

Is there a way to tell EF to not model this way and always include the join table? Or am I missing an easier way to create these join table records?

回答1:

You can create M:N relation in EF without retrieving objects as well:

using (var context = new MyContext())
{
   var firstEntity = new FirstEntity { Id = firstId };
   var secondEntity = new SecondEntity { Id = secondId; }

   context.FirstEntities.Attach(firstEntity);
   context.SecondEntities.Attach(secondEntity);

   firstEntity.SecondEntities = new HashSet<SecondEntity>();
   firstEntity.SecondEntities.Add(secondEntity);

   context.SaveChanges();
}

Anyway exposing junction table as entity is possible but you will lose comfort of EF and fallback to SQL like approach:

1. Delete M:N relation created by designer
2. Add new entity
3. Add two columns to the new entity representing foreign keys
4. Map the new entity to junction table
5. Add associations to related entities
6. Set referential constraints for added relations

回答2:

Entity Framework is an ORM, and as such, when you work with it you aren't suppose to think of the database in terms of tables but instead in terms of objects. You shouldn't be inserting the identity into a table that holds the M2M relationship, but you should be loading one side of the relationship, which should expose a collection of the other side and add it to that collection. For a M2M, you may need to load the other side and do the same.

Also, I believe EF prefers all tables to have a single column PK (I could be wrong on this), but you may need to add a column to the M2M and designate it as a PK.