Rails accepts_nested_attributes_for child doesn't have parent set when validating

Question

I\'m trying to access my parent model in my child model when validating. I found something about an inverse property on the has_one, but my Rails 2.3.5 doesn\'t recognize it, s

Answer 1

You cannot do this because in-memory child doesn't know the parent its assigned to. It only knows after save. For example.

child = parent.build_child
parent.child # => child
child.parent # => nil

# BUT
child.parent = parent
child.parent # => parent
parent.child # => child

So you can kind of force this behavior by doing reverse association manually. For example

def child_with_inverse_assignment=(child)
  child.parent = self
  self.child_without_inverse_assignment = child
end

def build_child_with_inverse_assignment(*args)
  build_child_without_inverse_assignment(*args)
  child.parent = self
  child
end

def create_child_with_inverse_assignment(*args)
  create_child_without_inverse_assignment(*args)
  child.parent = self
  child
end

alias_method_chain :"child=", :inverse_assignment
alias_method_chain :build_child, :inverse_assignment
alias_method_chain :create_child, :inverse_assignment

If you really find it necessary.

P.S. The reason it's not doing it now is because it's not too easy. It needs to be explicitly told how to access parent/child in each particular case. A comprehensive approach with identity map would've solved it, but for newer version there's :inverse_of workaround. Some discussions like this one took place on newsgroups.