self.available_columns = [
      QueryColumn.new(:project, :sortable => "#{Project.table_name}.name", :groupable => "#{Project.table_name}.name"),
      QueryColumn.new(:activity, :caption => :field_tt_booking_activity, :sortable => "#{TimeEntryActivity.table_name}.name", :groupable => "#{TimeEntryActivity.table_name}.name"),
      QueryColumn.new(:comments, :caption => :field_tt_comments),
      QueryColumn.new(:user, :sortable => "#{User.table_name}.login", :caption => :field_tt_user),