fix saving related with pk only model (#1297)

ormar/models/model.py

@@ -246,6 +246,7 @@ class Model(ModelRow):
         self_fields.pop(self.get_column_name_from_alias(self.ormar_config.pkname))
         if _columns:
             self_fields = {k: v for k, v in self_fields.items() if k in _columns}
+        if self_fields:
             self_fields = self.translate_columns_to_aliases(self_fields)
             expr = self.ormar_config.table.update().values(**self_fields)
             expr = expr.where(self.pk_column == getattr(self, self.ormar_config.pkname))

tests/test_model_methods/test_save_related_pk_only.py

@@ -0,0 +1,32 @@
+import ormar
+import pytest
+
+from tests.lifespan import init_tests
+from tests.settings import create_config
+
+base_ormar_config = create_config()
+
+
+class A(ormar.Model):
+    ormar_config = base_ormar_config.copy()
+
+    id = ormar.Integer(primary_key=True)
+
+
+class B(ormar.Model):
+    ormar_config = base_ormar_config.copy()
+
+    id = ormar.Integer(primary_key=True)
+    a = ormar.ForeignKey(A)
+
+
+create_test_database = init_tests(base_ormar_config)
+
+
+@pytest.mark.asyncio
+async def test_saving_related_pk_only():
+    async with base_ormar_config.database:
+        async with base_ormar_config.database.transaction(force_rollback=True):
+            a = A()
+            await a.save()
+            await a.save_related(follow=True, save_all=True)