Navigational attribute &1 is not part of PartProvider &2
A navigational attribute is not part of a PartProvider. If this is a warning, the navigational attribute data has to be read on-the-fly during execution of the query. If it is an error, on-the-fly processing is not possible.
The navigational attribute should be switched on at the corresponding PartProvider. Only then can the navigational attribute be read directly from the PartProvider. Otherwise, the on-the-fly processing takes place, not as good as the direct read from performance point of view.