test: add regression tests for issue #530 - relationship annotation preservation

Cover three scenarios:
1. Relationship annotations visible in __init_subclass__ hooks
2. Relationship fields excluded from Pydantic model_fields
3. End-to-end relationship functionality still works after the fix

Fixes #530

Author: aliyevr889

tests/test_relationship_annotation_preserved.py

@@ -0,0 +1,125 @@
+"""
+Regression test for issue #530:
+Relationship type annotations disappear after class definition is evaluated.
+
+Before the fix, `__init_subclass__` hooks could not see Relationship annotations
+in `cls.__annotations__`, making it impossible to inspect relationship types
+at class creation time.
+"""
+
+from typing import Optional
+
+from sqlmodel import Field, Relationship, SQLModel, Session, create_engine, select
+
+
+# Track what annotations were visible during class creation
+_seen_annotations: dict[str, set] = {}
+
+
+class AnnotationInspector(SQLModel):
+    """Mixin that records which annotations are visible in __init_subclass__."""
+
+    def __init_subclass__(cls, **kwargs: object) -> None:
+        super().__init_subclass__(**kwargs)
+        _seen_annotations[cls.__name__] = set(cls.__annotations__.keys())
+
+
+def test_relationship_annotations_visible_in_init_subclass() -> None:
+    """
+    Verifies that Relationship fields appear in __annotations__ when
+    __init_subclass__ is called, fixing issue #530.
+    """
+    _seen_annotations.clear()
+
+    class TeamA(AnnotationInspector, SQLModel, table=True):
+        __tablename__ = "teama_530"
+        id: Optional[int] = Field(default=None, primary_key=True)
+        members: list["MemberA"] = Relationship(back_populates="team")
+
+    class MemberA(AnnotationInspector, SQLModel, table=True):
+        __tablename__ = "membera_530"
+        id: Optional[int] = Field(default=None, primary_key=True)
+        team_id: Optional[int] = Field(default=None, foreign_key="teama_530.id")
+        team: Optional[TeamA] = Relationship(back_populates="members")
+
+    # The key assertion: relationship fields must be visible in __annotations__
+    # at the time __init_subclass__ is called.
+    assert "members" in _seen_annotations["TeamA"], (
+        "Relationship 'members' was not visible in TeamA.__annotations__ "
+        "during __init_subclass__ (issue #530)"
+    )
+    assert "team" in _seen_annotations["MemberA"], (
+        "Relationship 'team' was not visible in MemberA.__annotations__ "
+        "during __init_subclass__ (issue #530)"
+    )
+
+
+def test_relationship_annotations_not_in_model_fields() -> None:
+    """
+    Verifies that Relationship fields do NOT appear in model_fields (Pydantic),
+    which would cause validation overhead and incorrect behavior.
+    """
+
+    class TeamB(SQLModel, table=True):
+        __tablename__ = "teamb_530"
+        id: Optional[int] = Field(default=None, primary_key=True)
+        members: list["MemberB"] = Relationship(back_populates="team")
+
+    class MemberB(SQLModel, table=True):
+        __tablename__ = "memberb_530"
+        id: Optional[int] = Field(default=None, primary_key=True)
+        team_id: Optional[int] = Field(default=None, foreign_key="teamb_530.id")
+        team: Optional[TeamB] = Relationship(back_populates="members")
+
+    # Relationship fields should NOT appear in pydantic model_fields
+    assert "members" not in TeamB.model_fields, (
+        "Relationship 'members' incorrectly appeared in TeamB.model_fields"
+    )
+    assert "team" not in MemberB.model_fields, (
+        "Relationship 'team' incorrectly appeared in MemberB.model_fields"
+    )
+
+    # But they should appear in sqlmodel_relationships
+    assert "members" in TeamB.__sqlmodel_relationships__
+    assert "team" in MemberB.__sqlmodel_relationships__
+
+
+def test_relationship_functional_after_fix() -> None:
+    """
+    End-to-end test: Verify that relationships still work correctly after the fix.
+    """
+
+    class Department(SQLModel, table=True):
+        __tablename__ = "department_530"
+        id: Optional[int] = Field(default=None, primary_key=True)
+        name: str
+        employees: list["Employee"] = Relationship(back_populates="department")
+
+    class Employee(SQLModel, table=True):
+        __tablename__ = "employee_530"
+        id: Optional[int] = Field(default=None, primary_key=True)
+        name: str
+        department_id: Optional[int] = Field(
+            default=None, foreign_key="department_530.id"
+        )
+        department: Optional[Department] = Relationship(back_populates="employees")
+
+    engine = create_engine("sqlite://", echo=False)
+    SQLModel.metadata.create_all(engine)
+
+    with Session(engine) as session:
+        dept = Department(name="Engineering")
+        session.add(dept)
+        session.commit()
+        session.refresh(dept)
+
+        emp = Employee(name="Alice", department_id=dept.id)
+        session.add(emp)
+        session.commit()
+        session.refresh(emp)
+
+        # Verify relationship loading
+        statement = select(Employee).where(Employee.name == "Alice")
+        loaded_emp = session.exec(statement).first()
+        assert loaded_emp is not None
+        assert loaded_emp.department_id == dept.id