Using class variables like this will lead to the superclass having the [exact same value as this class](
http://www.railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/):

<pre>
[2] pry(main)> Queries::WorkPackages::Filter.filter_types_by_field
=> {:created_at=>:date_past,
 :updated_at=>:date_past,
 :status_id=>:list_status,
 :type_id=>:list,
 :priority_id=>:list,
 :subject=>:text,
 :start_date=>:date,
 :due_date=>:date,
 :estimated_hours=>:integer,
 :done_ratio=>:integer,
 :project_id=>:list,
 :category_id=>:list_optional,
 :fixed_version_id=>:list_optional,
 :subproject_id=>:list_subprojects,
 :assigned_to_id=>:list_optional,
 :author_id=>:list,
 :member_of_group=>:list_optional,
 :assigned_to_role=>:list_optional,
 :responsible_id=>:list_optional,
 :watcher_id=>:list}
[3] pry(main)> Queries::Filter.filter_types_by_field
=> {:created_at=>:date_past,
 :updated_at=>:date_past,
 :status_id=>:list_status,
 :type_id=>:list,
 :priority_id=>:list,
 :subject=>:text,
 :start_date=>:date,
 :due_date=>:date,
 :estimated_hours=>:integer,
 :done_ratio=>:integer,
 :project_id=>:list,
 :category_id=>:list_optional,
 :fixed_version_id=>:list_optional,
 :subproject_id=>:list_subprojects,
 :assigned_to_id=>:list_optional,
 :author_id=>:list,
 :member_of_group=>:list_optional,
 :assigned_to_role=>:list_optional,
 :responsible_id=>:list_optional,
 :watcher_id=>:list}
</pre>

I don't think that is intended.

Class variables should probably not be used here at all.