ruby on rails - Receiving already loaded object from find_by method of ActiveRecord::Associations::CollectionProx -

i have following models

class parent < activerecord::base   has_many :children end  class child < activerecord::base  belongs_to :parent end 

what want changing child object parent.children.find_or_initialize_by() , modify child object, save parent.save. children doesn't saved. here's code.

# prepare parent child p = parent.new({:name=>'parent'}) p.children << child.new({:name => 'child'}) p.save  # modify , save child p = parent.first c = p.children.find_or_initialize_by({:name=>'child'}) #=> returns child created above c.name = 'new child' p.save #=> children aren't saved 

i know because find_by method returns new object rather 1 loaded. seems more natural me if code above works because method invoked through activerecord::associations::collectionproxy. missing work? or there reasons find_by on activerecord::associations::collectionproxy have work does?

(i know can save modified child c.save, want save child through parant.save, because want errors on child object referenced parent.errors.)