Rails - Jun 2014 - Receiving already loaded object from find_by method of ActiveRecord::Associations::CollectionProx

I have the following models

    class Parent < ActiveRecord::Base
      has_many :children
    end

    class Child < ActiveRecord::Base
     belongs_to :parent
    end

What I want to do is changing a child object by 
parent.children.find_or_initialize_by() and modify the child object, then 
save it by parent.save. But the children doens't get saved. Here's the
code.

    # prepare a parent with a child
    p = Parent.new({:name=>'parent'})
    p.children << Child.new({:name => 'child'})
    p.save

    # modify and save child
    p = Parent.first
    c = p.children.find_or_initialize_by({:name=>'child'}) #=>
returns the
child already created above
    c.name = 'new child'
    p.save #=> children aren't saved

I know this is because find_by method always returns a new object rather 
than the one already loaded. But it seems more natural to me if the code 
above works because the method is invoked through 
ActiveRecord::Associations::CollectionProxy. Am I missing anything to get 
this to work? Or are there any reasons that find_by on 
ActiveRecord::Associations::CollectionProxy have to work as it currently 
does?

(I know I can save the modified child by c.save, but I want to save the 
child through parant.save, because I want the errors on child object to be 
referenced by parent.errors.)

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