Fixed PrivateAttr defaults ignored on database-loaded instances

sqlmodel/_compat.py

@@ -94,7 +94,20 @@ def get_model_fields(model: InstanceOrType[BaseModel]) -> dict[str, "FieldInfo"]
 def init_pydantic_private_attrs(new_object: InstanceOrType["SQLModel"]) -> None:
     object.__setattr__(new_object, "__pydantic_fields_set__", set())
     object.__setattr__(new_object, "__pydantic_extra__", None)
-    object.__setattr__(new_object, "__pydantic_private__", None)
+    # Initialize __pydantic_private__ with defaults from __private_attributes__,
+    # mirroring what Pydantic's own BaseModel.__init__ does. Previously this was
+    # set to None, which caused AttributeError when accessing PrivateAttr fields
+    # on instances reconstructed from the database (via __new__, bypassing __init__).
+    cls = new_object if isinstance(new_object, type) else new_object.__class__
+    private_attributes = getattr(cls, "__private_attributes__", {})
+    pydantic_private = {}
+    for k, v in private_attributes.items():
+        pydantic_private[k] = v.get_default()
+    object.__setattr__(
+        new_object,
+        "__pydantic_private__",
+        pydantic_private if pydantic_private else None,
+    )
 
 
 def get_annotations(class_dict: dict[str, Any]) -> dict[str, Any]:

tests/test_private_attr.py

@@ -0,0 +1,52 @@
+from pydantic import PrivateAttr
+from sqlmodel import Field, Session, SQLModel, create_engine, select
+
+
+def test_private_attr_default_preserved_after_db_load(clear_sqlmodel):
+    """PrivateAttr defaults should be available on instances loaded from DB (#149)."""
+
+    class Item(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        _secret: str = PrivateAttr(default="default_secret")
+
+    engine = create_engine("sqlite:///:memory:")
+    SQLModel.metadata.create_all(engine)
+
+    with Session(engine) as db:
+        item = Item(name="test")
+        assert item._secret == "default_secret"
+        db.add(item)
+        db.commit()
+
+    with Session(engine) as db:
+        loaded_item = db.exec(select(Item)).one()
+        # Previously raised AttributeError because __pydantic_private__ was None.
+        assert loaded_item._secret == "default_secret"
+
+    SQLModel.metadata.clear()
+
+
+def test_private_attr_default_factory_preserved_after_db_load(clear_sqlmodel):
+    """PrivateAttr with default_factory should work on DB-loaded instances (#149)."""
+
+    class Item(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        _tags: list[str] = PrivateAttr(default_factory=list)
+
+    engine = create_engine("sqlite:///:memory:")
+    SQLModel.metadata.create_all(engine)
+
+    with Session(engine) as db:
+        item = Item(name="test")
+        item._tags.append("hello")
+        db.add(item)
+        db.commit()
+
+    with Session(engine) as db:
+        loaded_item = db.exec(select(Item)).one()
+        # Should get a fresh empty list from default_factory, not AttributeError.
+        assert loaded_item._tags == []
+
+    SQLModel.metadata.clear()