How to eager load objects with a custom join in rails?

Can you not add the join conditions using ActiveRecord?

For example, I have a quite complex query using several dependent records and it works fine by combining conditions and include directives

Contractors.find(
  :all, 
  :include => {:council_areas => :suburbs},
  :conditions => ["suburbs.postcode = ?", customer.postcode]                 
)    

Assuming that:

1. Contractors have_many CouncilAreas
2. CouncilAreas have_many Suburbs

This join returns the Contractors in the suburb identified by customer.postcode.

The generated query looks like:

SELECT contractors.*, council_areas.*, suburbs.*
FROM `contractors` 
LEFT OUTER JOIN `contractors_council_areas` ON `contractors_council_areas`.contractor_id = `contractors`.id 
LEFT OUTER JOIN `council_areas` ON `council_areas`.id = `contractors_council_areas`.council_area_id 
LEFT OUTER JOIN `council_areas_suburbs` ON `council_areas_suburbs`.council_area_id = `council_areas`.id 
LEFT OUTER JOIN `suburbs` ON `suburbs`.id = `council_areas_suburbs`.suburb_id WHERE (suburbs.postcode="5000")

(Note: I edited the column list for brevity).