has_one with a belongs_to as source association

This patches allow you to use a has_one association with a belongs_to as source. Assuming that House belongs to City that belongs to Country:

  class House
    belongs_to :city
    has_one :country, :through => :city
  end

  class City
    belongs_to :country
  end

  class Country
  end

The following works in Rails current versions (preload case):

  House.all(:include => :country)

The following DOES NOT work on Rails current version and is fixed by this patch (non-preload case):

  House.all(:include => :country, :conditions => ["countries.name = ?", "Brazil"])

However, as I talked with Pratik and Koz on IRC, we could simply do this:

  House.all(:include => { :city => :country }, :conditions => ["countries.name = ?", "Brazil"])

I've created the patch because I've needed this behavior in geokit-rails, which was later fixed to work with the { :city => :country } case, but currently it has no use and it should not be applied until someone comes up with a use case for it.

This ticket is just to document the patch.