I've got an error in tests due to the fact this file is not called in controller.

You should add something like 
    respond_to do |format|
      format.json { render :json => @target_assignment }
    end
in use_another_assignment_groups in groups_controller.rb.

But, this won't be enought. Have a look to tests (you can run `bundle exec ruby test/functional/groups_controller_test.rb`)