feat: add lateral_load() for efficient Many:One eager loading via JOIN LATERAL

tests/test_crud.py

@@ -6,9 +6,15 @@ import pytest
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
-from fastapi_toolsets.crud import CrudFactory, PaginationType
-from fastapi_toolsets.crud.factory import AsyncCrud, _CursorDirection
+from fastapi_toolsets.crud import CrudFactory, PaginationType, lateral_load
+from fastapi_toolsets.crud.factory import (
+    AsyncCrud,
+    _CursorDirection,
+    _LateralLoad,
+    _ResolvedLateral,
+)
 from fastapi_toolsets.exceptions import NotFoundError
+from fastapi_toolsets.schemas import PydanticBase
 
 from .conftest import (
     EventCreate,
@@ -51,6 +57,12 @@ from .conftest import (
 )
 
 
+class UserWithRoleRead(PydanticBase):
+    id: uuid.UUID
+    username: str
+    role: RoleRead | None = None
+
+
 class TestCrudFactory:
     """Tests for CrudFactory."""
 
@@ -208,11 +220,11 @@ class TestResolveLoadOptions:
         assert crud._resolve_load_options(None) is None
 
     def test_empty_list_overrides_default(self):
-        """An empty list is a valid override and disables default_load_options."""
+        """An explicit empty list disables default_load_options (no options applied)."""
         default = [selectinload(User.role)]
         crud = CrudFactory(User, default_load_options=default)
-        # Empty list is not None, so it should replace default
-        assert crud._resolve_load_options([]) == []
+        # Empty list replaces default; None and [] are both falsy → no options applied
+        assert not crud._resolve_load_options([])
 
 
 class TestResolveSearchColumns:
@@ -359,13 +371,6 @@ class TestDefaultLoadOptionsIntegration:
         self, db_session: AsyncSession
     ):
         """default_load_options loads relationships automatically on offset_paginate()."""
-        from fastapi_toolsets.schemas import PydanticBase
-
-        class UserWithRoleRead(PydanticBase):
-            id: uuid.UUID
-            username: str
-            role: RoleRead | None = None
-
         UserWithDefaultLoad = CrudFactory(
             User, default_load_options=[selectinload(User.role)]
         )
@@ -2462,12 +2467,7 @@ class TestCursorPaginateExtraOptions:
     @pytest.mark.anyio
     async def test_with_load_options(self, db_session: AsyncSession):
         """cursor_paginate passes load_options to the query."""
-        from fastapi_toolsets.schemas import CursorPagination, PydanticBase
-
-        class UserWithRoleRead(PydanticBase):
-            id: uuid.UUID
-            username: str
-            role: RoleRead | None = None
+        from fastapi_toolsets.schemas import CursorPagination
 
         role = await RoleCrud.create(db_session, RoleCreate(name="manager"))
         for i in range(3):
@@ -2833,3 +2833,368 @@ class TestPaginate:
 
         assert isinstance(result.pagination, OffsetPagination)
         assert result.pagination.total_count is None
+
+
+class TestLateralLoadValidation:
+    """lateral_load() raises immediately for bad relationship types."""
+
+    def test_valid_many_to_one_returns_marker(self):
+        """lateral_load() on a Many:One rel returns a _LateralLoad with rel_attr set."""
+        marker = lateral_load(User.role)
+        assert isinstance(marker, _LateralLoad)
+        assert marker.rel_attr is User.role
+
+    def test_raises_type_error_for_plain_column(self):
+        """lateral_load() raises TypeError when passed a plain column."""
+        with pytest.raises(TypeError, match="relationship attribute"):
+            lateral_load(User.username)
+
+    def test_raises_value_error_for_many_to_many(self):
+        """lateral_load() raises ValueError for Many:Many (secondary table)."""
+        with pytest.raises(ValueError, match="Many:Many"):
+            lateral_load(Post.tags)
+
+    def test_raises_value_error_for_one_to_many(self):
+        """lateral_load() raises ValueError for One:Many (uselist=True)."""
+        with pytest.raises(ValueError, match="One:Many"):
+            lateral_load(Role.users)
+
+
+class TestLateralLoadInSubclass:
+    """lateral_load() markers in default_load_options are processed at class definition."""
+
+    def test_marker_extracted_from_default_load_options(self):
+        """_LateralLoad is removed from default_load_options and stored in _resolved_lateral."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        assert UserLateralCrud.default_load_options is None
+        assert UserLateralCrud._resolved_lateral is not None
+
+    def test_resolved_lateral_has_one_join_and_eager(self):
+        """_resolved_lateral contains exactly one join and one eager option."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        resolved = UserLateralCrud._resolved_lateral
+        assert isinstance(resolved, _ResolvedLateral)
+        assert len(resolved.joins) == 1
+        assert len(resolved.eager) == 1
+
+    def test_regular_options_preserved_alongside_lateral(self):
+        """Non-lateral opts stay in default_load_options; lateral marker is extracted."""
+        regular = selectinload(User.role)
+
+        class UserMixedCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role), regular]
+
+        assert UserMixedCrud._resolved_lateral is not None
+        assert UserMixedCrud.default_load_options == [regular]
+
+    def test_no_lateral_leaves_default_load_options_untouched(self):
+        """When no lateral marker is present, default_load_options is unchanged."""
+        opts = [selectinload(User.role)]
+
+        class UserNormalCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = opts
+
+        assert UserNormalCrud.default_load_options is opts
+        assert UserNormalCrud._resolved_lateral is None
+
+    def test_no_default_load_options_leaves_resolved_lateral_none(self):
+        """_resolved_lateral stays None when default_load_options is not set."""
+
+        class UserPlainCrud(AsyncCrud[User]):
+            model = User
+
+        assert UserPlainCrud._resolved_lateral is None
+
+
+class TestResolveLoadOptionsWithLateral:
+    """_resolve_load_options always appends lateral eager options."""
+
+    def test_lateral_eager_included_when_no_call_site_opts(self):
+        """contains_eager from lateral_load is returned when load_options=None."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        resolved = UserLateralCrud._resolve_load_options(None)
+        assert resolved is not None
+        assert len(resolved) == 1  # the contains_eager
+
+    def test_lateral_eager_appended_to_call_site_opts(self):
+        """call-site load_options + lateral eager are both returned."""
+        extra = selectinload(User.role)
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        resolved = UserLateralCrud._resolve_load_options([extra])
+        assert resolved is not None
+        assert len(resolved) == 2
+
+    def test_lateral_eager_appended_to_default_load_options(self):
+        """default_load_options (regular) + lateral eager are both returned."""
+        regular = selectinload(User.role)
+
+        class UserMixedCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role), regular]
+
+        resolved = UserMixedCrud._resolve_load_options(None)
+        assert resolved is not None
+        assert len(resolved) == 2
+
+
+class TestGetLateralJoins:
+    """_get_lateral_joins merges auto-resolved and manual lateral_joins."""
+
+    def test_returns_none_when_no_lateral_configured(self):
+        """Returns None when neither lateral_joins nor lateral_load is set."""
+
+        class UserPlainCrud(AsyncCrud[User]):
+            model = User
+
+        assert UserPlainCrud._get_lateral_joins() is None
+
+    def test_returns_resolved_lateral_joins(self):
+        """Returns the join tuple built from lateral_load()."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        joins = UserLateralCrud._get_lateral_joins()
+        assert joins is not None
+        assert len(joins) == 1
+
+    def test_manual_lateral_joins_included(self):
+        """Manual lateral_joins class var is included in _get_lateral_joins."""
+        from sqlalchemy import select, true
+
+        manual_sub = select(Role).where(Role.id == User.role_id).lateral("_manual_role")
+
+        class UserManualCrud(AsyncCrud[User]):
+            model = User
+            lateral_joins = [(manual_sub, true())]
+
+        joins = UserManualCrud._get_lateral_joins()
+        assert joins is not None
+        assert len(joins) == 1
+
+    def test_manual_and_auto_lateral_joins_merged(self):
+        """Both manual lateral_joins and auto-resolved from lateral_load are combined."""
+        from sqlalchemy import select, true
+
+        manual_sub = select(Role).where(Role.id == User.role_id).lateral("_manual_role")
+
+        class UserBothCrud(AsyncCrud[User]):
+            model = User
+            lateral_joins = [(manual_sub, true())]
+            default_load_options = [lateral_load(User.role)]
+
+        joins = UserBothCrud._get_lateral_joins()
+        assert joins is not None
+        assert len(joins) == 2
+
+
+class TestLateralLoadIntegration:
+    """lateral_load() in real DB queries: relationship loaded, pagination correct."""
+
+    @pytest.mark.anyio
+    async def test_get_loads_relationship(self, db_session: AsyncSession):
+        """get() populates the relationship via lateral join."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="admin"))
+        user = await UserCrud.create(
+            db_session,
+            UserCreate(username="alice", email="alice@test.com", role_id=role.id),
+        )
+
+        fetched = await UserLateralCrud.get(db_session, [User.id == user.id])
+        assert fetched.role is not None
+        assert fetched.role.name == "admin"
+
+    @pytest.mark.anyio
+    async def test_get_null_fk_preserved(self, db_session: AsyncSession):
+        """User with null role_id still returned (LEFT JOIN behaviour)."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        user = await UserCrud.create(
+            db_session, UserCreate(username="bob", email="bob@test.com")
+        )
+
+        fetched = await UserLateralCrud.get(db_session, [User.id == user.id])
+        assert fetched is not None
+        assert fetched.role is None
+
+    @pytest.mark.anyio
+    async def test_first_loads_relationship(self, db_session: AsyncSession):
+        """first() populates the relationship via lateral join."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="editor"))
+        await UserCrud.create(
+            db_session,
+            UserCreate(username="carol", email="carol@test.com", role_id=role.id),
+        )
+
+        user = await UserLateralCrud.first(db_session)
+        assert user is not None
+        assert user.role is not None
+        assert user.role.name == "editor"
+
+    @pytest.mark.anyio
+    async def test_get_multi_loads_relationship(self, db_session: AsyncSession):
+        """get_multi() populates the relationship via lateral join for all rows."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="member"))
+        for i in range(3):
+            await UserCrud.create(
+                db_session,
+                UserCreate(
+                    username=f"user{i}", email=f"u{i}@test.com", role_id=role.id
+                ),
+            )
+
+        users = await UserLateralCrud.get_multi(db_session)
+        assert len(users) == 3
+        assert all(u.role is not None and u.role.name == "member" for u in users)
+
+    @pytest.mark.anyio
+    async def test_offset_paginate_correct_count(self, db_session: AsyncSession):
+        """offset_paginate total_count is not inflated by the lateral join."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="admin"))
+        for i in range(5):
+            await UserCrud.create(
+                db_session,
+                UserCreate(
+                    username=f"user{i}", email=f"u{i}@test.com", role_id=role.id
+                ),
+            )
+
+        result = await UserLateralCrud.offset_paginate(
+            db_session, schema=UserWithRoleRead, items_per_page=10
+        )
+        assert result.pagination.total_count == 5
+        assert len(result.data) == 5
+
+    @pytest.mark.anyio
+    async def test_offset_paginate_loads_relationship(self, db_session: AsyncSession):
+        """offset_paginate serializes relationship data loaded via lateral."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="admin"))
+        await UserCrud.create(
+            db_session,
+            UserCreate(username="alice", email="alice@test.com", role_id=role.id),
+        )
+
+        result = await UserLateralCrud.offset_paginate(
+            db_session, schema=UserWithRoleRead, items_per_page=10
+        )
+        assert result.data[0].role is not None
+        assert result.data[0].role.name == "admin"
+
+    @pytest.mark.anyio
+    async def test_offset_paginate_mixed_null_fk(self, db_session: AsyncSession):
+        """offset_paginate returns all users including those with null role_id."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="admin"))
+        await UserCrud.create(
+            db_session,
+            UserCreate(username="with_role", email="a@test.com", role_id=role.id),
+        )
+        await UserCrud.create(
+            db_session, UserCreate(username="no_role", email="b@test.com")
+        )
+
+        result = await UserLateralCrud.offset_paginate(
+            db_session, schema=UserWithRoleRead, items_per_page=10
+        )
+        assert result.pagination.total_count == 2
+
+    @pytest.mark.anyio
+    async def test_cursor_paginate_loads_relationship(self, db_session: AsyncSession):
+        """cursor_paginate populates the relationship via lateral join."""
+
+        class UserLateralCursorCrud(AsyncCrud[User]):
+            model = User
+            cursor_column = User.id
+            default_load_options = [lateral_load(User.role)]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="admin"))
+        for i in range(3):
+            await UserCrud.create(
+                db_session,
+                UserCreate(
+                    username=f"user{i}", email=f"u{i}@test.com", role_id=role.id
+                ),
+            )
+
+        result = await UserLateralCursorCrud.cursor_paginate(
+            db_session, schema=UserWithRoleRead, items_per_page=10
+        )
+        assert len(result.data) == 3
+        assert all(item.role is not None for item in result.data)
+
+    @pytest.mark.anyio
+    async def test_offset_paginate_with_search_and_lateral(
+        self, db_session: AsyncSession
+    ):
+        """search filter works alongside lateral join."""
+
+        class UserLateralCrud(AsyncCrud[User]):
+            model = User
+            default_load_options = [lateral_load(User.role)]
+            searchable_fields = [User.username]
+
+        role = await RoleCrud.create(db_session, RoleCreate(name="admin"))
+        await UserCrud.create(
+            db_session,
+            UserCreate(username="alice", email="a@test.com", role_id=role.id),
+        )
+        await UserCrud.create(
+            db_session, UserCreate(username="bob", email="b@test.com", role_id=role.id)
+        )
+
+        result = await UserLateralCrud.offset_paginate(
+            db_session, schema=UserWithRoleRead, search="alice", items_per_page=10
+        )
+        assert result.pagination.total_count == 1
+        assert result.data[0].username == "alice"