The query.select_related has numerous states: it can be False (for no select_related), it can be True (which seems to indicate that depth based select_related should be used) or it can be a dict (for user-selected restriction). I think here field_dict could end up being True here.

In the long run it would be better to split the query.select_related to two variables: one would be query.select_related = True/False, another would ne query.select_related_restriction = None or dict_of_asked_selections.

For the time being maybe
field_dict = isinstance(self.select_related, dict) and self.select_related or {}
would do? (naturally, not tested).

So the question now is: what to do when select_related is True? Two use cases:

# will work ok
Species.objects.select_related(depth=4).select_related('genus') 

# should get all immediate relations AND the Order
Species.objects.select_related(depth=1).select_related('genus__family__order') 

I seems to me we need this split on select_related and we need to handle it intelligently, ie. 'count' depth of select related field and conditionally add it or not.