fix: update pk field only raises unfriendly error

CHANGELOG.rst

@@ -6,22 +6,20 @@ Changelog
 
 .. rst-class:: emphasize-children
 
-0.25
+0.24
 ====
 
-0.25.0 (unreleased)
+0.24.1 (unreleased)
 ------
 Fixed
 ^^^^^
 - Fixed asyncio "no current event loop" deprecation warning by replacing `asyncio.get_event_loop()` with modern event loop handling using `get_running_loop()` with fallback to `new_event_loop()` (#1865)
+- Fix update pk field only raises unfriendly error (#1873)
 
 Changed
 ^^^^^^^
 - add benchmarks for `get_for_dialect` (#1862)
 
-0.24
-====
-
 0.24.0
 ------
 Fixed

tests/test_model_methods.py

@@ -1,6 +1,8 @@
 import os
 from uuid import uuid4
 
+import pytest
+
 from tests.testmodels import (
     Dest_null,
     Event,
@@ -121,6 +123,8 @@ async def test_save_partial(self):
         n_mdl = await self.cls.get(id=self.mdl.id)
         self.assertEqual(n_mdl.name, "Test")
         self.assertEqual(n_mdl.desc, "Something")
+        with pytest.raises(OperationalError):
+            await self.mdl.save(update_fields=["id"])
 
     async def test_create(self):
         mdl = self.cls(name="Test2")

tortoise/backends/base/executor.py

@@ -267,13 +267,15 @@ async def execute_update(
         values = []
         expressions = {}
         for field in update_fields or self.model._meta.fields_db_projection.keys():
-            if not self.model._meta.fields_map[field].pk:
-                instance_field = getattr(instance, field)
-                if isinstance(instance_field, Expression):
+            if not (field_obj := self.model._meta.fields_map[field]).pk:
+                if isinstance(instance_field := getattr(instance, field), Expression):
                     expressions[field] = instance_field
                 else:
-                    value = self.model._meta.fields_map[field].to_db_value(instance_field, instance)
-                    values.append(value)
+                    values.append(field_obj.to_db_value(instance_field, instance))
+        if not values and not expressions:
+            raise OperationalError(
+                f"Can't update pk field only, use `{self.model.__name__}.create` instead."
+            )
         values.append(self.model._meta.pk.to_db_value(instance.pk, instance))
         return (
             await self.db.execute_query(self.get_update_sql(update_fields, expressions), values)