accepts_nested_attributes_for not updating attributes

In Rails 2.3.8 I discovered that accepts_nested_attributes_for may skip updating existing records under certain circumstances. I pulled my hair out yesterday because of this: In the application it didn't update my nested objects, reproducing this on the console updated the objects. It took me some hours to discover the difference (after finding many reports for similar bugs and workaround/solutions which didn't work for me):

In the application I did

class TestController < ApplicationController
  # class Container belongs_to :location; end
  # class Location has_many :containers; accepts_nested_attributes_for :containers, :allow_destroy => true; end

  def update
    @location = Location.find(params[:id])
    if @location.update_attributes params[:location]
      #...
    end
  end
end

This never worked, although the :containers_attributes hash was correctly passed in by the params hash. Only new records were added to the association. Existing records couldn't be updated or destroyed.

To investigate I copy/pasted the hash from the log to the console doing the following way:

$ script/console
> l = Location.find(123)
> l.containers.inspect   #<-- to see if the prerequisites for the following hash are met
> l.update_attributes(:containers_attributes => { ... })  #<-- copied from the development.log
> l.reload.containers.inspect   #<-- compare: it worked!

Now what was the difference? After a few hours of google'ing around it not easy to still have a view for that, but finally I discovered the small difference:

In my controller I didn't load the containers association prior to updating it while on the console I did. Adding :include => :containers to the Location finder fixed all my problems.

I think it is a bug in Rails that it does not implicitly do that if the association has not been loaded upon mass assignment to Location#containers_attributes=. It should be fixed.