Normalize to strings to handle form encoded data

src/documents/serialisers.py

@@ -714,15 +714,18 @@ class BatchedManyRelatedField(serializers.ManyRelatedField):
             pks.append(item)
 
         try:
-            found = {obj.pk: obj for obj in child.get_queryset().filter(pk__in=pks)}
+            # str() normalizes int vs string PKs — form data sends strings, JSON sends ints
+            found = {
+                str(obj.pk): obj for obj in child.get_queryset().filter(pk__in=pks)
+            }
         except (TypeError, ValueError):
             child.fail("incorrect_type", data_type=type(pks[0]).__name__)
 
         result = []
         for pk in pks:
-            if pk not in found:
+            if str(pk) not in found:
                 child.fail("does_not_exist", pk_value=pk)
-            result.append(found[pk])
+            result.append(found[str(pk)])
         return result