improve code structure

sqlmodel/_compat.py

@@ -66,6 +66,29 @@ def _is_union_type(t: Any) -> bool:
 finish_init: ContextVar[bool] = ContextVar("finish_init", default=True)
 
 
+def set_polymorphic_default_value(self_instance, values):
+    """By defalut, when init a model, pydantic will set the polymorphic_on
+    value to field default value. But when inherit a model, the polymorphic_on
+    should be set to polymorphic_identity value by default."""
+    cls = type(self_instance)
+    mapper = inspect(cls)
+    if isinstance(mapper, Mapper):
+        polymorphic_on = mapper.polymorphic_on
+        if polymorphic_on is not None:
+            polymorphic_property = mapper.get_property_by_column(polymorphic_on)
+            field_info = get_model_fields(cls).get(polymorphic_property.key)
+            if field_info:
+                v = values.get(polymorphic_property.key)
+                # if model is inherited or polymorphic_on is not explicitly set
+                # set the polymorphic_on by default
+                if mapper.inherits or v is None:
+                    setattr(
+                        self_instance,
+                        polymorphic_property.key,
+                        mapper.polymorphic_identity,
+                    )
+
+
 @contextmanager
 def partial_init() -> Generator[None, None, None]:
     token = finish_init.set(False)
@@ -293,22 +316,7 @@ def sqlmodel_table_construct(
                 setattr(self_instance, key, value)
         # End SQLModel override
         # Override polymorphic_on default value
-        mapper = inspect(cls)
-        if isinstance(mapper, Mapper):
-            polymorphic_on = mapper.polymorphic_on
-            if polymorphic_on is not None:
-                polymorphic_property = mapper.get_property_by_column(polymorphic_on)
-                field_info = cls.model_fields.get(polymorphic_property.key)
-                if field_info:
-                    v = values.get(polymorphic_property.key)
-                    # if model is inherited or polymorphic_on is not explicitly set
-                    # set the polymorphic_on by default
-                    if mapper.inherits or v is None:
-                        setattr(
-                            self_instance,
-                            polymorphic_property.key,
-                            mapper.polymorphic_identity,
-                        )
+        set_polymorphic_default_value(self_instance, values)
         return self_instance
 
     def sqlmodel_validate(
@@ -592,3 +600,5 @@ def sqlmodel_init(*, self: "SQLModel", data: Dict[str, Any]) -> None:
             for key in non_pydantic_keys:
                 if key in self.__sqlmodel_relationships__:
                     setattr(self, key, data[key])
+        # Override polymorphic_on default value
+        set_polymorphic_default_value(self, values)

tests/test_polymorphic_model.py

@@ -51,7 +51,7 @@ class DarkHero(Hero):
 
 
 @needs_pydanticv2
-def test_polymorphic_joined_table_sm_field(clear_sqlmodel) -> None:
+def test_polymorphic_joined_table_with_sqlmodel_field(clear_sqlmodel) -> None:
     class Hero(SQLModel, table=True):
         __tablename__ = "hero"
         id: Optional[int] = Field(default=None, primary_key=True)
@@ -123,7 +123,7 @@ class DarkHero(Hero):
     with Session(engine) as db:
         hero = Hero()
         db.add(hero)
-        dark_hero = DarkHero()
+        dark_hero = DarkHero(dark_power="pokey")
         db.add(dark_hero)
         db.commit()
         statement = select(DarkHero)