gistfile1.rb

require 'rubygems'
require 'dm-core' #0.9.11

DataMapper.setup(:default, 'sqlite3::memory:')

class Location
  include DataMapper::Resource
  property :id, Serial
  property :city, String
end
class Distance
  include DataMapper::Resource
  property :id, Serial
  property :amount, Integer
  belongs_to :origin, :class_name => "Location", :child_key => [:origin_location_id]
  belongs_to :destination, :class_name => "Location", :child_key => [:destination_location_id]
end

DataMapper.auto_migrate!
Location.create(:city => "Houston")
Location.create(:city => "Chicago")
Distance.create(:amount => 1000, :origin_location_id => 1, :destination_location_id => 2)
Distance.first("origin.city" => "Houston", "destination.city" => "Chicago")

gistfile2.txt

/Library/Ruby/Gems/1.8/gems/dm-core-0.9.11/lib/dm-core/adapters/data_objects_adapter.rb:66:in `execute_reader': ambiguous column name: distances.id (Sqlite3Error)
Query: SELECT "distances"."id", "distances"."amount", "distances"."origin_location_id",
"distances"."destination_location_id" FROM "distances" INNER JOIN "locations" ON ("locations"."id" =
"distances"."destination_location_id") INNER JOIN "distances" ON ("locations"."id" =
"distances"."origin_location_id") WHERE ("locations"."city" = 'Chicago') AND ("locations"."city" = 'Houston')
ORDER BY "distances"."id" LIMIT 1
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.9.11/lib/dm-core/adapters/data_objects_adapter.rb:66:in `read_one'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.9.11/lib/dm-core/adapters/data_objects_adapter.rb:173:in `with_connection'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.9.11/lib/dm-core/adapters/data_objects_adapter.rb:61:in `read_one'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.9.11/lib/dm-core/repository.rb:72:in `read_one'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.9.11/lib/dm-core/model.rb:267:in `first'
	from finder.rb:23