ActiveRecord changed? flag doesn't report changes in associated children

Question

I've got the following situation with ActiveRecord (in Rails 2.3.8):

class Order < ActiveRecord::Base
  has_many :documents
  accepts_nested_attributes_for :documents
end

class Document <ActiveRecord::Base
  belongs_to :order
end

Now, in a controller I want to direct the user differently depending on whether or not they made changes to existing records, e.g.

@order.attributes = params[:order]
if @order.changed?
  # save order
  # redirect one place
else
  # redirect another place
end

For that I want to use the changed? flag. It turns out that @order.changed? does not query the children.

I tried to implement that via an association proxy method on the has_many :documents association, like so:

  has_many :documents do
    def changed?
      any? {|doc| doc.changed?}
    end
  end

But this has the unintended side-effect that it loads the associated documents from disk, which apparently wipes out any changes that were made in the nested attributes assignment in @order.attributes = params[:order] This either seems an intentional design decision in Rails, but then how to solve it? Or is it a feature gap?

Thoughts?

Wolf

Answer 1

Hi,

you might have a look at the source of nested_records_changed_for_autosave.

It is not exactly what you want, but it has clues about how to do it. Especially association_instance_get which "Gets the specified association instance if it responds to :loaded?, nil otherwise."