app/routing.yml
In this example, my Model is identified by A string + an id.
That's why I specified requirements.

attached_file:
  class: sfDoctrineRouteCollection
  options:
    model:        AttachedFile
    module:       attachment
    prefix_path:  /attachment
    column:       obj_type/:obj_pk
    with_wildcard_routes: true
    requirements:
      obj_type: [a-zA-z-_]
      obj_pk: \d+