drf-writable-nested's update_or_create_reverse_relations rebuilds a
fresh serializer -- and fresh field instances -- per custom_fields item
while matching existing vs. new instances during save(), so the
per-instance lookup cache alone only helped the first validation pass.
It passes the same context dict (by reference) to every one of those
serializers, so stash resolved CustomField objects there instead:
later passes reuse them for free rather than re-querying.
Trenton Holmes
0a466c9fcf