# File lib/thinking_sphinx/association.rb, line 102
    def primary_key_from_reflection
      if @reflection.options[:through]
        @reflection.source_reflection.options[:foreign_key] ||
        @reflection.source_reflection.primary_key_name
      elsif @reflection.macro == :has_and_belongs_to_many
        @reflection.association_foreign_key
      else
        nil
      end
    end