Fix deduplicating books that are on a shelf or in a list

Previously when the deduplicate_book_data script tried to merge an
edition that was on a shelf or in a list then it would fail because when
the canonical book was added to the shelf or the list then it wouldn’t
set the extra fields of the linking table for the “through” model of the
field. These would end up defaulting to NULL, but that is not valid for
some of the fields in ShelfItem and ListItem so postgres wouldn’t accept
it.

To fix that, this patch makes it skip updating fields that have a
non-autogenerated linking table. The linking table would appear as a
separate model anyway so the book will be moved via that instead.

Fixes: #2817

bookwyrm/management/commands/deduplicate_book_data.py

@@ -1,7 +1,7 @@
 """ PROCEED WITH CAUTION: uses deduplication fields to permanently
 merge book data objects """
 from django.core.management.base import BaseCommand
-from django.db.models import Count
+from django.db.models import Count, ManyToManyField
 from bookwyrm import models
 
 
@@ -12,6 +12,16 @@ def update_related(canonical, obj):
         (r.remote_field.name, r.related_model) for r in canonical._meta.related_objects
     ]
     for (related_field, related_model) in related_models:
+        # Skip the ManyToMany fields that aren’t auto-created. These
+        # should have a corresponding OneToMany field in the model for
+        # the linking table anyway. If we update it through that model
+        # instead then we won’t lose the extra fields in the linking
+        # table.
+        related_field_obj = related_model._meta.get_field(related_field)
+        if isinstance(related_field_obj, ManyToManyField):
+            through = related_field_obj.remote_field.through
+            if not through._meta.auto_created:
+                continue
         related_objs = related_model.objects.filter(**{related_field: obj})
         for related_obj in related_objs:
             print("replacing in", related_model.__name__, related_field, related_obj.id)