gistfile1.txt

class Ad
  include DataMapper::Resource
  property :id,                 Integer,          :serial => true
  has n,  :counties, :class_name => 'Ads::Counties', :child_key => [:ad_id], :parent_key => [:id]
end

module Ads
  class Counties
    include DataMapper::Resource
    property :id,              Serial
    property :ad_id,           Integer,   :index => true
    property :county_name,     String,    :index => true

    belongs_to :ad, :class_name => 'Ad', :child_key => [:ad_id]
  end
end # Ads

Here is the error trace:
NoMethodError: undefined method `counties_association' for #<Ads::Counties id=1 ad_id=1 county_name=nil>
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:153:in `send'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:153:in `get_parent'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:152:in `each'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:152:in `get_parent'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:168:in `with_repository'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/support/kernel.rb:6:in `repository'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core.rb:181:in `repository'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/repository.rb:44:in `scope'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core.rb:181:in `repository'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/support/kernel.rb:6:in `repository'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:168:in `with_repository'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:131:in `get_parent'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:95:in `get_children'
	from /opt/local/lib/ruby/gems/1.8/gems/extlib-0.9.8/lib/extlib/lazy_array.rb:14:in `each'
	from /opt/local/lib/ruby/gems/1.8/gems/extlib-0.9.8/lib/extlib/lazy_array.rb:14:in `each'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/relationship.rb:94:in `get_children'
... 8 levels...
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/one_to_many.rb:242:in `children'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.6/lib/dm-core/associations/one_to_many.rb:300:in `method_missing'
	from /opt/local/lib/ruby/1.8/irb.rb:302:in `output_value'
	from /opt/local/lib/ruby/1.8/irb.rb:151:in `eval_input'
	from /opt/local/lib/ruby/1.8/irb.rb:263:in `signal_status'
	from /opt/local/lib/ruby/1.8/irb.rb:147:in `eval_input'
	from /opt/local/lib/ruby/1.8/irb.rb:146:in `eval_input'
	from /opt/local/lib/ruby/1.8/irb.rb:70:in `start'
	from /opt/local/lib/ruby/1.8/irb.rb:69:in `catch'
	from /opt/local/lib/ruby/1.8/irb.rb:69:in `start'
	from /opt/local/lib/ruby/gems/1.8/gems/merb-core-0.9.7/lib/merb-core/rack/adapter/irb.rb:106:in `start'
	from /opt/local/lib/ruby/gems/1.8/gems/merb-core-0.9.7/lib/merb-core/server.rb:54:in `start'
	from /opt/local/lib/ruby/gems/1.8/gems/merb-core-0.9.7/lib/merb-core.rb:108:in `start'
	from /opt/local/lib/ruby/gems/1.8/gems/merb-core-0.9.7/bin/merb:11
	from /opt/local/bin/merb:19:in `load'
	from /opt/local/bin/merb:19irb