Fix instance of related object added to session on validation

Previously, when validating instance when a session was open and the
model instance had a related object a new instance of this related
object was created and added to the session.

sqlmodel/_compat.py

@@ -335,7 +335,7 @@ def sqlmodel_validate(
             for key in new_obj.__sqlmodel_relationships__:
                 value = getattr(use_obj, key, Undefined)
                 if value is not Undefined:
-                    setattr(new_obj, key, value)
+                    new_obj.__dict__[key] = value
         return new_obj
 
     def sqlmodel_init(*, self: "SQLModel", data: Dict[str, Any]) -> None:

tests/test_validation.py

@@ -1,8 +1,9 @@
-from typing import Optional
+from typing import List, Optional
 
 import pytest
 from pydantic.error_wrappers import ValidationError
-from sqlmodel import SQLModel
+from sqlmodel import Session, SQLModel, create_engine
+from sqlmodel.main import Field, Relationship
 
 from .conftest import needs_pydanticv1, needs_pydanticv2
 
@@ -63,3 +64,71 @@ def reject_none(cls, v):
 
     with pytest.raises(ValidationError):
         Hero.model_validate({"name": None, "age": 25})
+
+
+@needs_pydanticv1
+def test_validation_related_object_not_in_session_pydantic_v1(clear_sqlmodel):
+    class Team(SQLModel, table=True):
+        id: Optional[int] = Field(default=None, primary_key=True)
+        name: str
+        heroes: List["Hero"] = Relationship(back_populates="team")
+
+    class Hero(SQLModel, table=True):
+        id: Optional[int] = Field(default=None, primary_key=True)
+        name: str
+
+        team_id: Optional[int] = Field(default=None, foreign_key="team.id")
+        team: Optional[Team] = Relationship(back_populates="heroes")
+
+    engine = create_engine("sqlite://")
+    SQLModel.metadata.create_all(engine)
+    team = Team(name="team")
+    hero = Hero(name="hero", team=team)
+    with Session(engine) as session:
+        session.add(team)
+        session.add(hero)
+        session.commit()
+
+    with Session(engine) as session:
+        hero = session.get(Hero, 1)
+        assert not session.dirty
+        assert not session.new
+
+        Hero.validate(hero)
+
+        assert not session.dirty
+        assert not session.new
+
+
+@needs_pydanticv2
+def test_validation_related_object_not_in_session_pydantic_v2(clear_sqlmodel):
+    class Team(SQLModel, table=True):
+        id: Optional[int] = Field(default=None, primary_key=True)
+        name: str
+        heroes: List["Hero"] = Relationship(back_populates="team")
+
+    class Hero(SQLModel, table=True):
+        id: Optional[int] = Field(default=None, primary_key=True)
+        name: str
+
+        team_id: Optional[int] = Field(default=None, foreign_key="team.id")
+        team: Optional[Team] = Relationship(back_populates="heroes")
+
+    engine = create_engine("sqlite://")
+    SQLModel.metadata.create_all(engine)
+    team = Team(name="team")
+    hero = Hero(name="hero", team=team)
+    with Session(engine) as session:
+        session.add(team)
+        session.add(hero)
+        session.commit()
+
+    with Session(engine) as session:
+        hero = session.get(Hero, 1)
+        assert not session.dirty
+        assert not session.new
+
+        Hero.model_validate(hero)
+
+        assert not session.dirty
+        assert not session.new