Support editing a many_many on a has_one object in parent form

[jthomerson]

If you have the following model:

• Parent has_one Child named Child
• Child many_many OtherThing named OtherThings

and you want to edit the Child<-many_many->OtherThing relationship in your Parent edit form, you can now do so by passing $this (parent) as the owner and "Child.OtherThings" as the name of the field.