如何使用Entity Framework Code First Fluent API仅在连接表上级联删除

Question

I'm trying to map an existing database using Entity Framework Code First Fluent API. I'm struggling on a situation with a many-to-many relationship and cascade delete.

Here's my two POCO classes:

public class Foo
{
  public int FooId;
  public string FooDescription;
  public IList<Bar> Bars;
}
public class Bar
{
  public int BarId;
  public string BarDescription;
}

For now, I only have a navigation property from Foo to Bar . That's because in my real scenario, it doesn't make sense for anyone to go from Bar to Foo . The relationship from Foo to Bar is actually 0,n which means one instance of Foo do not require instances of Bar .

My existing database has a join table between those two tables. This table looks like this:

create table Foo_Bar(
FooId int not null,
BarId int not null
);

Because I prefer to not create such a POCO class in my project, I mapped the table like this:

modelBuilder.Entity<Foo>()
  .HasMany(t => t.Bars)
  .WithMany()
  .Map(m =>
  {
    m.ToTable("Foo_Bar");
    m.MapLeftKey("FooId");
    m.MapRightKey("BarId");
  });

That works perfectly for inserts. Entity insert on Foo and then on the existing Foo_Bar table.

But when I delete a Foo , I would like to delete the records on the join table but not on the child table. Meaning I want to delete records on Foo_Bar but not to extend this cascade to Bar .

Is such a mapping possible? I should mention that on my context I removed both OneToManyCascadeDeleteConvention and ManyToManyCascadeDeleteConvention because 99% of the times I won't be using cascade delete. This is one specific situation.

By tracing the SQL generated by Entity Framework, I also noticed that if I retrieve a Foo instance and set all Bars inside it to EntityState.Deleted, EF will delete both records in Foo_Bar and Bar . One last thing I should mention is that I'm working in a disconnected scenario.

I tried to search this over but it seems I'm not using the right keywords as I could not find anyone in such a situation. At this point the only solution I see is to Map a FooBar class but I'd hope there's a way to do this.

--Edit

It appears that my approach to delete was wrong at first. When I tried deleting I wasn't passing the instance of Foo with its list of Bar filled. After that change, Entity Framwork creates a delete statement for every Bar on the list. I still have problems with updating like adding and removing items at the same time. Will do more tests and post here.

Thanks in advance.

Answer 1

But when I delete a Foo, I would like to delete the records on the join table but not on the child table. Meaning I want to delete records on Foo_Bar but not to extend this cascade to Bar.

Is such a mapping possible?

This is actually the only mapping that is possible. You cannot extend cascading delete to the Bar table because in the relationship (from database schema perspective) the Foo_Bar join table is the dependent in the relationship and Bar is the principal . Generally cacscading delete is only in effect if you delete the principal but not when you delete the dependent.

So, you don't need to worry that a Bar could be deleted when you delete a Foo and you don't have to apply any special setting for this.

But you have a problem when you have removed the ManyToManyCascadeDeleteConvention . It acts globally for the whole model and there is no option to turn on cascading delete again for a specific many-to-many relationship. (There is no WillCascadeOnDelete(true) for many-to-many relationship like there is one for one-to-many or one-to-one relationships.)

Are you sure that removing the ManyToManyCascadeDeleteConvention is really required in your model? I never encountered a model where it would make sense to not delete the related entries in the join table when one of the joined entities ( Foo or Bar ) is deleted.