Failing test can be found here:
http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/17...

Eager loading of has_one :through has an easy to fix problem.  It
doesn't pass the :conditions or :order of the base association down to
the "source" include on the :through association.  has_many :through
does this already, the "options" just need to be passed down.

The larger problem, of both has_one :through and has_many :through is
when you have a :conditions that references the :through table.  Since
preload never does any joining, it always does the associations
independently, it throws a SQL error because it can't find that table.

This is only a problem with eager loading, the find_every code in
base.rb does the correct detection normally.

On 9 Jan 2009, at 20:46, Joseph Palermo wrote:

I've been vaguely meaning to rewrite the preload code so that it loads  
hmt associations in one go (rather than loading the records in two  
steps).
There's a preliminary task which is to refactor the hmt code to make  
it easy for the preload stuff to recreate the appropriate joins (don't  
really fancy duplicating all that like in join based eager loading).

Fred