Fix issue with relationship dict

sqlmodel/_compat.py

@@ -320,7 +320,11 @@ def sqlmodel_validate(
     # Get and set any relationship objects
     if is_table_model_class(cls):
         for key in new_obj.__sqlmodel_relationships__:
-            value = getattr(use_obj, key, Undefined)
+            value = (
+                use_obj.get(key, Undefined)
+                if isinstance(use_obj, dict)
+                else getattr(use_obj, key, Undefined)
+            )
             if value is not Undefined:
                 setattr(new_obj, key, value)
     return new_obj

tests/test_relationship_dict.py

@@ -0,0 +1,67 @@
+"""
+Regression test for: model_validate with a dict containing relationship keys
+silently drops the relationship data.
+
+Root cause: sqlmodel_validate in _compat.py uses getattr(use_obj, key, Undefined)
+to extract relationship values. When use_obj is a dict, getattr finds no attribute
+named e.g. "heroes", returning Undefined, so relationships are never set.
+
+Fix: use dict.get() when use_obj is a dict instance.
+"""
+
+from sqlalchemy.orm import selectinload
+from sqlmodel import Field, Relationship, Session, SQLModel, create_engine, select
+
+
+def test_model_validate_dict_populates_relationships():
+    """Relationships passed as dict keys to model_validate should be set."""
+
+    class Team(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        heroes: list["Hero"] = Relationship(back_populates="team")
+
+    class Hero(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        team_id: int | None = Field(default=None, foreign_key="team.id")
+        team: Team | None = Relationship(back_populates="heroes")
+
+    heroes = [Hero(name="Deadpond"), Hero(name="Spider-Boy")]
+    team = Team.model_validate({"name": "Z-Force", "heroes": heroes})
+
+    assert len(team.heroes) == 2, (
+        "model_validate with a dict should populate relationship fields"
+    )
+
+
+def test_model_validate_dict_relationships_persisted():
+    """Relationships set via model_validate dict should be saved and loadable."""
+
+    class Team(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        heroes: list["Hero"] = Relationship(back_populates="team")
+
+    class Hero(SQLModel, table=True):
+        id: int | None = Field(default=None, primary_key=True)
+        name: str
+        team_id: int | None = Field(default=None, foreign_key="team.id")
+        team: Team | None = Relationship(back_populates="heroes")
+
+    engine = create_engine("sqlite://")
+    SQLModel.metadata.create_all(engine)
+
+    heroes = [Hero(name="Deadpond"), Hero(name="Spider-Boy")]
+    team = Team.model_validate({"name": "Z-Force", "heroes": heroes})
+
+    with Session(engine) as session:
+        session.add(team)
+        session.commit()
+
+    with Session(engine) as session:
+        loaded = session.exec(
+            select(Team).options(selectinload(Team.heroes))  # type: ignore[arg-type]
+        ).first()
+        assert loaded is not None
+        assert len(loaded.heroes) == 2