     getParticipants: (id) ->
       Promise.resolve()
       .then =>
+        validateId(id)
         @storageClient.getObjectMetadata(id)
       .then (metadata) ->
         usernames = _.chain([metadata.owners, metadata.readers, metadata.writers])

    visit: (id) ->
      Promise.resolve()
      .then =>
        @changePermissions(id)
      .then =>
        @changed.push(id)


      Promise.resolve()
      .then =>
        validateId(id)
        @getParticipants(id)
      .then (current) =>
        usersAdd    = _.difference(@users, current)
