fix: update pk field only raises unfriendly error

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

@@ -21,6 +21,7 @@
 
 if TYPE_CHECKING:  # pragma: nocoverage
     from tortoise.backends.base.client import BaseDBAsyncClient
+    from tortoise.fields.base import Field
     from tortoise.models import Model
     from tortoise.query_utils import Prefetch
     from tortoise.queryset import QuerySet
@@ -264,16 +265,20 @@ def get_update_sql(
     async def execute_update(
         self, instance: "Union[type[Model], Model]", update_fields: Optional[Iterable[str]]
     ) -> int:
+        fields: "dict[str, Field]" = {
+            field: field_obj
+            for field in update_fields or self.model._meta.fields_db_projection
+            if not (field_obj := self.model._meta.fields_map[field]).pk
+        }
+        if not fields:
+            raise OperationalError(f"Can't update pk field only, use `{self.model.__name__}.create` instead.")
         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):
-                    expressions[field] = instance_field
-                else:
-                    value = self.model._meta.fields_map[field].to_db_value(instance_field, instance)
-                    values.append(value)
+        for field, field_obj in fields.items():
+            if isinstance(instance_field := getattr(instance, field), Expression):
+                expressions[field] = instance_field
+            else:
+                values.append(field_obj.to_db_value(instance_field, instance))
         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)