- The :class:`.Query` will raise an exception when :meth:`.Query.yield_per`
is used with mappings or options where eager loading, either
joined or subquery, would take place.  These loading strategies are
not currently compatible with yield_per, so by raising this error,
the method is safer to use - combine with sending False to
:meth:`.Query.enable_eagerloads` to disable the eager loaders.