Allow empty fields to bulk_update_with_history

This fixes a "hack" for when a user only wants to provide the
`custom_historical_attrs` argument, where a list with arbitrary field
names could be provided as a circumvention for the error that was raised
otherwise - even if no model fields were actually changed.

CHANGES.rst

@@ -10,6 +10,8 @@ Unreleased
 - Added ``custom_historical_attrs`` to ``bulk_create_with_history()`` and
   ``bulk_update_with_history()`` for setting additional fields on custom history models
   (gh-1248)
+- Passing an empty list as the ``fields`` argument to ``bulk_update_with_history()`` is
+  now allowed; history records will still be created (gh-1248)
 
 
 3.4.0 (2023-08-18)

docs/common_issues.rst

@@ -119,7 +119,7 @@ A field ``session`` would be passed as ``custom_historical_attrs={'session': 'ja
 .. code-block:: pycon
 
     >>> bulk_update_with_history(
-            data, PollWithHistoricalSessionAttr,
+            data, PollWithHistoricalSessionAttr, [],
             custom_historical_attrs={'session': 'jam'}
         )
     >>> data[0].history.latest().session

simple_history/tests/tests/test_utils.py

@@ -541,7 +541,7 @@ def test_bulk_update_history_with_custom_model_attributes(self):
         bulk_update_with_history(
             self.data,
             PollWithHistoricalSessionAttr,
-            fields=["question"],
+            fields=[],
             custom_historical_attrs={"session": "training"},
         )
 

simple_history/utils.py

@@ -169,7 +169,8 @@ def bulk_update_with_history(
     their history (all in one transaction).
     :param objs: List of objs of type model to be updated
     :param model: Model class that should be updated
-    :param fields: The fields that are updated
+    :param fields: The fields that are updated. If empty, no model objects will be
+        changed, but history records will still be created.
     :param batch_size: Number of objects that should be updated in each batch
     :param default_user: Optional user to specify as the history_user in each historical
         record
@@ -189,7 +190,15 @@ def bulk_update_with_history(
         raise AlternativeManagerError("The given manager does not belong to the model.")
 
     with transaction.atomic(savepoint=False):
-        rows_updated = model_manager.bulk_update(objs, fields, batch_size=batch_size)
+        if not fields:
+            # Allow not passing any fields if the user wants to bulk-create history
+            # records - e.g. with `custom_historical_attrs` provided
+            # (Calling `bulk_update()` with no fields would have raised an error)
+            rows_updated = 0
+        else:
+            rows_updated = model_manager.bulk_update(
+                objs, fields, batch_size=batch_size
+            )
         history_manager.bulk_history_create(
             objs,
             batch_size=batch_size,