Ticket #35399: group_the_case-whens_into_sub-chunks_.patch

django/db/models/query.py

@@ -5 +5 @@
 import copy
 import operator
 import warnings
+from collections import defaultdict
 from itertools import chain, islice
 
 from asgiref.sync import sync_to_async
@@ -907 +908 @@
             update_kwargs = {}
             for field in fields:
                 when_statements = []
+                chunks = defaultdict(list)
                 for obj in batch_objs:
                     attr = getattr(obj, field.attname)
                     if not hasattr(attr, "resolve_expression"):
                         attr = Value(attr, output_field=field)
+                    try:
+                        if chunks is not None:
+                            chunks[attr].append(obj.pk)
+                    except TypeError:
+                        # one value can not be hashed, so omit chunking
+                        chunks = None
                     when_statements.append(When(pk=obj.pk, then=attr))
-                case_statement = Case(*when_statements, output_field=field)
+                if chunks is not None:  # all values were hashable
+                    case_statement = Case(*[When(pk__in=pks, then=val) for val, pks in chunks.items()], output_field=field)
+                else:
+                    case_statement = Case(*when_statements, output_field=field)
                 if requires_casting:
                     case_statement = Cast(case_statement, output_field=field)
                 update_kwargs[field.attname] = case_statement