https://github.com/theforeman/foreman/blob/develop/app/controllers/api/base_controller.rb?source=cc#L35

def resource_class
      @resource_class ||= resource_name.classify.constantize
    end

When katello engine api controllers extend Foreman::Api::V2::BaseController and attempt to use the base helper methods such as

before_filter :find_resource

the 'resource_class' fails because it cannot find the class.

If the foreman core is going to truly support engines, this needs to be fixed.