We had an odd bug last week where we ended up with different results after we had eager loaded an association vs loaded directly.
There are apparently two issues with :has_one :through, one of which also applies to :has_many :through.
class Person has_many :friendships has_one :best_friend, :through => :friendships, :conditions => "friendships.best = 1" end
If you do a Person.find(:all, :include => :best_friend), the best_friend that gets preloaded is not necessarily one that has a “friendship.best = 1″
This is due to a bug in the association preloading code that doesn’t pass down the finder options, so any :conditions or :order are completely ignored. This problem is easy to fix, just a one line change, but it then exposes another problem.
This problem applies to both :has_many :through and :has_one :through associations. The problem is that the :through association is loaded separately from the :has_one or :has_many association. So it first loads :friendships, and then when it tries to load :best_friend, it doesn’t have the table it needs for the :conditions and explodes.
Our current work around is basically putting the conditions on the :through association, although sometimes you need to create a new association just for that which is certainly not idea, especially if you plan on accessing the :through model after it has been loaded.
The way to fix it in Rails is unfortunately a rewrite of how the :through associations are eager loaded.