Fixed PrivateAttr defaults lost when loading from database (#149)

When SQLAlchemy reconstructs a model instance from a database query
(bypassing __init__), it goes through __new__ which calls
init_pydantic_private_attrs(). That function was setting
__pydantic_private__ = None instead of initializing it with the
private attribute defaults. This caused AttributeError when accessing
PrivateAttr fields on database-loaded instances.

The fix introspects the class's __private_attributes__ dict and calls
get_default() on each entry, mirroring what Pydantic's own __init__
does.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: xr843

sqlmodel/_compat.py

@@ -94,7 +94,18 @@ 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__).
+    pydantic_private = {}
+    for k, v in new_object.__class__.__private_attributes__.items():
+        pydantic_private[k] = v.get_default(call_default_factory=True)
+    object.__setattr__(
+        new_object,
+        "__pydantic_private__",
+        pydantic_private if pydantic_private else None,
+    )
 
 
 def get_annotations(class_dict: dict[str, Any]) -> dict[str, Any]: