product.rb

class Product < ActiveRecord::Base
  def self.supplied_within_states(*states)
    # So you can pass either an array or a list of arguments to the method
    states = states.flatten(1)

    if states.include? 'all'
      scoped
    else
      # Here be dragons! - RS 2013-05-20
      # Ideally this query would be much simpler - something like:
      # scoped.joins(:product_variants => {:supplier_product_variants => :supplier}).merge(Supplier.in_states(states))
      # to join all the tables and then filter on the final supplier table... however!
      # between Rails 3.0 not supporting nested has_many through associations (which is where the supplier table comes from)
      # and `merge` behaving a bit strangely with said nested HMT (support for this got added by a gem).. this is the final result
      # It takes a couple of queries (two here and several in the Supplier.in_states class method) but it does the job
      # When this app is upgraded to Rails 3.1+ this can be revisited (as it supports nested HMT natively
      # and probably has a bunch of other fixes too)
      scoped.where(:id => joins( :product_variants => { :supplier_product_variants => :supplier } ).where(:suppliers => {:id => Supplier.in_states(states).select(:id)}))
    end
  end
end