Allows you to specify the model association key in a belongs_to
relationship instead of the foreign key.
The following queries are now equivalent:
Post.where(:author_id => Author.first)
Post.where(:author => Author.first)
PriceEstimate.where(:estimate_of_type => 'Treasure', :estimate_of_id => treasure)
PriceEstimate.where(:estimate_of => treasure)
