Support join based eagerloading of has_one through associations

(from [6ae0a0557d5e2859e359275b5feebb7e3c13271c]) Load the first and not the last has_one result when doing join-based eager loading

This matters when the has_one is defined with an order in which case there is an expectation that the first one will be loaded.

[#904 state:resolved]

Signed-off-by: Jeremy Kemper jeremy@bitsweat.net http://github.com/rails/rails/co...

Hi,

we just got bitten by the change done in http://github.com/rails/rails/commit/a445cdd

This breaks nested has_many associations as illustrated in ticket #5623.

As we understand it this patch was added to cover cases like this (taken from post.rb test model):

has_one :last_comment, :class_name => 'Comment', :order => 'id desc'

Well, this seems like an abuse of the has_one association and can definitely be written in better ways, which is something we should be promoting, like:

has_many :comments do
  def last
   find(:first, :order => "id DESC")
  end
end

One could argue that providing an :order option for has_one relations doesn't make any sense at all. A has_one denotes that there is only one record on the other side of the association and "ordering" it doesn't look right. Perhaps tho it serves another purpose we seem to neglect...

We propose a reversal of the patch done in http://github.com/rails/rails/commit/a445cdd, since it breaks legitimate use cases (in a way that it's easy to miss it returns the wrong results), while there's still a way to write the above scenario in a clean, workable way.

Any insight on this issue is mostly welcomed.
-fotos