Use all column names in GROUP BY for Postgres (Pascal Betz).

generators/tagging/templates/tagging_extensions.rb

@@ -125,11 +125,16 @@ def tagged_with(*tag_list)
       sql << "AND taggings.taggable_type = '#{ActiveRecord::Base.send(:class_name_of_active_record_descendant, self).to_s}' "
       sql << "AND taggings.tag_id = tags.id "
 
-      tag_list_condition = tag_list.map {|t| "'#{t}'"}.join(", ")
+      tag_list_condition = tag_list.map {|name| "'#{name}'"}.join(", ")
 
       sql << "AND (tags.name IN (#{sanitize_sql(tag_list_condition)})) "
       sql << "AND #{sanitize_sql(options[:conditions])} " if options[:conditions]
-      sql << "GROUP BY #{table_name}.id "
+
+      columns = column_names.map do |column| 
+        "#{table_name}.#{column}"
+      end.join(", ")
+
+      sql << "GROUP BY #{columns} "
       sql << "HAVING COUNT(taggings.tag_id) = #{tag_list.size}"
 
       add_order!(sql, options[:order], scope)