diff --git a/sqlmodel/_compat.py b/sqlmodel/_compat.py
index 4e80cdc374..6a6012ee91 100644
--- a/sqlmodel/_compat.py
+++ b/sqlmodel/_compat.py
@@ -341,7 +341,7 @@ 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:
diff --git a/tests/test_validation.py b/tests/test_validation.py
index 3265922070..92f57c5d5e 100644
--- a/tests/test_validation.py
+++ b/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 +64,73 @@ 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 session._is_clean()
+
+        new_hero = Hero.validate(hero)
+
+        assert session._is_clean()
+        # The new hero is a different instance, but the team is the same
+        assert id(new_hero) != id(hero)
+        assert id(new_hero.team) == id(hero.team)
+
+
+@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 session._is_clean()
+
+        new_hero = Hero.model_validate(hero)
+
+        assert session._is_clean()
+        # The new hero is a different instance, but the team is the same
+        assert id(new_hero) != id(hero)
+        assert id(new_hero.team) == id(hero.team)