# File lib/thinking_sphinx/source/sql.rb, line 87
      def sql_group_clause
        internal_groupings = []
        if @model.column_names.include?(@model.inheritance_column)
           internal_groupings << "#{@model.quoted_table_name}.#{quote_column(@model.inheritance_column)}"
        end

        (
          ["#{@model.quoted_table_name}.#{quote_column(@model.primary_key_for_sphinx)}"] + 
          @fields.collect     { |field|     field.to_group_sql     }.compact +
          @attributes.collect { |attribute| attribute.to_group_sql }.compact +
          @groupings + internal_groupings
        ).join(", ")
      end