feat: add M2M helpers (#247)

docs/module/db.md

@@ -118,6 +118,57 @@ async def clean(db_session):
     await cleanup_tables(session=db_session, base=Base)
 ```
 
+## Many-to-Many helpers
+
+SQLAlchemy's ORM collection API triggers lazy-loads when you append to a relationship inside a savepoint (e.g. inside `lock_tables` or a nested `get_transaction`). The three `m2m_*` helpers bypass the ORM collection entirely and issue direct SQL against the association table.
+
+### `m2m_add` — insert associations
+
+[`m2m_add`](../reference/db.md#fastapi_toolsets.db.m2m_add) inserts one or more rows into a secondary table without touching the ORM collection:
+
+```python
+from fastapi_toolsets.db import lock_tables, m2m_add
+
+async with lock_tables(session, [Tag]):
+    tag = await TagCrud.create(session, TagCreate(name="python"))
+    await m2m_add(session, post, Post.tags, tag)
+```
+
+Pass `ignore_conflicts=True` to silently skip associations that already exist:
+
+```python
+await m2m_add(session, post, Post.tags, tag, ignore_conflicts=True)
+```
+
+### `m2m_remove` — delete associations
+
+[`m2m_remove`](../reference/db.md#fastapi_toolsets.db.m2m_remove) deletes specific association rows. Removing a non-existent association is a no-op:
+
+```python
+from fastapi_toolsets.db import get_transaction, m2m_remove
+
+async with get_transaction(session):
+    await m2m_remove(session, post, Post.tags, tag1, tag2)
+```
+
+### `m2m_set` — replace the full set
+
+[`m2m_set`](../reference/db.md#fastapi_toolsets.db.m2m_set) atomically replaces all associations: it deletes every existing row for the owner instance then inserts the new set. Passing no related instances clears the association entirely:
+
+```python
+from fastapi_toolsets.db import get_transaction, m2m_set
+
+# Replace all tags
+async with get_transaction(session):
+    await m2m_set(session, post, Post.tags, tag_a, tag_b)
+
+# Clear all tags
+async with get_transaction(session):
+    await m2m_set(session, post, Post.tags)
+```
+
+All three helpers raise `TypeError` if the relationship attribute is not a Many-to-Many (i.e. has no secondary table).
+
 ---
 
 [:material-api: API Reference](../reference/db.md)

docs/reference/db.md

@@ -13,6 +13,9 @@ from fastapi_toolsets.db import (
     create_db_context,
     get_transaction,
     lock_tables,
+    m2m_add,
+    m2m_remove,
+    m2m_set,
     wait_for_row_change,
 )
 ```
@@ -32,3 +35,9 @@ from fastapi_toolsets.db import (
 ## ::: fastapi_toolsets.db.create_database
 
 ## ::: fastapi_toolsets.db.cleanup_tables
+
+## ::: fastapi_toolsets.db.m2m_add
+
+## ::: fastapi_toolsets.db.m2m_remove
+
+## ::: fastapi_toolsets.db.m2m_set

src/fastapi_toolsets/db.py

@@ -4,11 +4,13 @@ import asyncio
 from collections.abc import AsyncGenerator, Callable
 from contextlib import AbstractAsyncContextManager, asynccontextmanager
 from enum import Enum
-from typing import Any, TypeVar
+from typing import Any, TypeVar, cast
 
-from sqlalchemy import text
+from sqlalchemy import Table, delete, text, tuple_
+from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
-from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy.orm import DeclarativeBase, QueryableAttribute
+from sqlalchemy.orm.relationships import RelationshipProperty
 
 from .exceptions import NotFoundError
 
@@ -20,6 +22,9 @@ __all__ = [
     "create_db_dependency",
     "get_transaction",
     "lock_tables",
+    "m2m_add",
+    "m2m_remove",
+    "m2m_set",
     "wait_for_row_change",
 ]
 
@@ -339,3 +344,140 @@ async def wait_for_row_change(
         current = {col: getattr(instance, col) for col in watch_cols}
         if current != initial:
             return instance
+
+
+def _m2m_prop(rel_attr: QueryableAttribute) -> RelationshipProperty:  # type: ignore[type-arg]
+    """Return the validated M2M RelationshipProperty for *rel_attr*.
+
+    Raises TypeError if *rel_attr* is not a Many-to-Many relationship.
+    """
+    prop = rel_attr.property
+    if not isinstance(prop, RelationshipProperty) or prop.secondary is None:
+        raise TypeError(
+            f"m2m helpers require a Many-to-Many relationship attribute, "
+            f"got {rel_attr!r}. Use a relationship with a secondary table."
+        )
+    return prop
+
+
+async def m2m_add(
+    session: AsyncSession,
+    instance: DeclarativeBase,
+    rel_attr: QueryableAttribute,
+    *related: DeclarativeBase,
+    ignore_conflicts: bool = False,
+) -> None:
+    """Insert rows into a Many-to-Many association table without loading the ORM collection.
+
+    Args:
+        session: DB async session.
+        instance: The "owner" side model instance (e.g. the ``A`` in ``A.b_list``).
+        rel_attr: The M2M relationship attribute on the model class (e.g. ``A.b_list``).
+        *related: One or more related instances to associate with ``instance``.
+        ignore_conflicts: When ``True``, silently skip rows that already exist
+            in the association table (``ON CONFLICT DO NOTHING``).
+
+    Raises:
+        TypeError: If ``rel_attr`` is not a Many-to-Many relationship.
+    """
+    prop = _m2m_prop(rel_attr)
+    if not related:
+        return
+
+    secondary = cast(Table, prop.secondary)
+    assert secondary is not None  # guaranteed by _m2m_prop
+    sync_pairs = prop.secondary_synchronize_pairs
+    assert sync_pairs is not None  # set whenever secondary is set
+
+    # synchronize_pairs:           [(parent_col, assoc_col), ...]
+    # secondary_synchronize_pairs: [(related_col, assoc_col), ...]
+    rows: list[dict[str, Any]] = []
+    for rel_instance in related:
+        row: dict[str, Any] = {}
+        for parent_col, assoc_col in prop.synchronize_pairs:
+            row[assoc_col.name] = getattr(instance, cast(str, parent_col.key))
+        for related_col, assoc_col in sync_pairs:
+            row[assoc_col.name] = getattr(rel_instance, cast(str, related_col.key))
+        rows.append(row)
+
+    stmt = pg_insert(secondary).values(rows)
+    if ignore_conflicts:
+        stmt = stmt.on_conflict_do_nothing()
+    await session.execute(stmt)
+
+
+async def m2m_remove(
+    session: AsyncSession,
+    instance: DeclarativeBase,
+    rel_attr: QueryableAttribute,
+    *related: DeclarativeBase,
+) -> None:
+    """Remove rows from a Many-to-Many association table without loading the ORM collection.
+
+    Args:
+        session: DB async session.
+        instance: The "owner" side model instance (e.g. the ``A`` in ``A.b_list``).
+        rel_attr: The M2M relationship attribute on the model class (e.g. ``A.b_list``).
+        *related: One or more related instances to disassociate from ``instance``.
+
+    Raises:
+        TypeError: If ``rel_attr`` is not a Many-to-Many relationship.
+    """
+    prop = _m2m_prop(rel_attr)
+    if not related:
+        return
+
+    secondary = cast(Table, prop.secondary)
+    assert secondary is not None  # guaranteed by _m2m_prop
+    related_pairs = prop.secondary_synchronize_pairs
+    assert related_pairs is not None  # set whenever secondary is set
+
+    parent_where = [
+        assoc_col == getattr(instance, cast(str, parent_col.key))
+        for parent_col, assoc_col in prop.synchronize_pairs
+    ]
+
+    if len(related_pairs) == 1:
+        related_col, assoc_col = related_pairs[0]
+        related_values = [getattr(r, cast(str, related_col.key)) for r in related]
+        related_where = assoc_col.in_(related_values)
+    else:
+        assoc_cols = [ac for _, ac in related_pairs]
+        rel_cols = [rc for rc, _ in related_pairs]
+        related_values_t = [
+            tuple(getattr(r, cast(str, rc.key)) for rc in rel_cols) for r in related
+        ]
+        related_where = tuple_(*assoc_cols).in_(related_values_t)
+
+    await session.execute(delete(secondary).where(*parent_where, related_where))
+
+
+async def m2m_set(
+    session: AsyncSession,
+    instance: DeclarativeBase,
+    rel_attr: QueryableAttribute,
+    *related: DeclarativeBase,
+) -> None:
+    """Replace the entire Many-to-Many association set atomically.
+
+    Args:
+        session: DB async session.
+        instance: The "owner" side model instance (e.g. the ``A`` in ``A.b_list``).
+        rel_attr: The M2M relationship attribute on the model class (e.g. ``A.b_list``).
+        *related: The new complete set of related instances.
+
+    Raises:
+        TypeError: If ``rel_attr`` is not a Many-to-Many relationship.
+    """
+    prop = _m2m_prop(rel_attr)
+    secondary = cast(Table, prop.secondary)
+    assert secondary is not None  # guaranteed by _m2m_prop
+
+    parent_where = [
+        assoc_col == getattr(instance, cast(str, parent_col.key))
+        for parent_col, assoc_col in prop.synchronize_pairs
+    ]
+    await session.execute(delete(secondary).where(*parent_where))
+
+    if related:
+        await m2m_add(session, instance, rel_attr, *related)

tests/test_db.py

@@ -4,10 +4,26 @@ import asyncio
 import uuid
 
 import pytest
-from sqlalchemy import text
+from sqlalchemy import (
+    Column,
+    ForeignKey,
+    ForeignKeyConstraint,
+    String,
+    Table,
+    Uuid,
+    select,
+    text,
+)
 from sqlalchemy.engine import make_url
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
-from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy.orm import (
+    DeclarativeBase,
+    Mapped,
+    mapped_column,
+    relationship,
+    selectinload,
+)
 
 from fastapi_toolsets.db import (
     LockMode,
@@ -17,12 +33,15 @@ from fastapi_toolsets.db import (
     create_db_dependency,
     get_transaction,
     lock_tables,
+    m2m_add,
+    m2m_remove,
+    m2m_set,
     wait_for_row_change,
 )
 from fastapi_toolsets.exceptions import NotFoundError
 from fastapi_toolsets.pytest import create_db_session
 
-from .conftest import DATABASE_URL, Base, Role, RoleCrud, User, UserCrud
+from .conftest import DATABASE_URL, Base, Post, Role, RoleCrud, Tag, User, UserCrud
 
 
 class TestCreateDbDependency:
@@ -81,6 +100,21 @@ class TestCreateDbDependency:
 
         await engine.dispose()
 
+    @pytest.mark.anyio
+    async def test_no_commit_when_not_in_transaction(self):
+        """Dependency skips commit if the session is no longer in a transaction on exit."""
+        engine = create_async_engine(DATABASE_URL, echo=False)
+        session_factory = async_sessionmaker(engine, expire_on_commit=False)
+        get_db = create_db_dependency(session_factory)
+
+        async for session in get_db():
+            # Manually commit — session exits the transaction
+            await session.commit()
+            assert not session.in_transaction()
+            # The dependency's post-yield path must not call commit again (no error)
+
+        await engine.dispose()
+
     @pytest.mark.anyio
     async def test_update_after_lock_tables_is_persisted(self):
         """Changes made after lock_tables exits (before endpoint returns) are committed.
@@ -480,3 +514,417 @@ class TestCleanupTables:
         async with create_db_session(DATABASE_URL, Base, drop_tables=True) as session:
             # Should not raise
             await cleanup_tables(session, EmptyBase)
+
+
+class TestM2MAdd:
+    """Tests for m2m_add helper."""
+
+    @pytest.mark.anyio
+    async def test_adds_single_related(self, db_session: AsyncSession):
+        """Associates one related instance via the secondary table."""
+        user = User(username="m2m_author", email="m2m@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post A", author_id=user.id)
+        tag = Tag(name="python")
+        db_session.add_all([post, tag])
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags, tag)
+
+        result = await db_session.execute(
+            select(Post).where(Post.id == post.id).options(selectinload(Post.tags))
+        )
+        loaded = result.scalar_one()
+        assert len(loaded.tags) == 1
+        assert loaded.tags[0].id == tag.id
+
+    @pytest.mark.anyio
+    async def test_adds_multiple_related(self, db_session: AsyncSession):
+        """Associates multiple related instances in a single call."""
+        user = User(username="m2m_author2", email="m2m2@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post B", author_id=user.id)
+        tag1 = Tag(name="web")
+        tag2 = Tag(name="api")
+        tag3 = Tag(name="async")
+        db_session.add_all([post, tag1, tag2, tag3])
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags, tag1, tag2, tag3)
+
+        result = await db_session.execute(
+            select(Post).where(Post.id == post.id).options(selectinload(Post.tags))
+        )
+        loaded = result.scalar_one()
+        assert {t.id for t in loaded.tags} == {tag1.id, tag2.id, tag3.id}
+
+    @pytest.mark.anyio
+    async def test_noop_for_empty_related(self, db_session: AsyncSession):
+        """Calling with no related instances is a no-op."""
+        user = User(username="m2m_author3", email="m2m3@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post C", author_id=user.id)
+        db_session.add(post)
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags)  # no related instances
+
+        result = await db_session.execute(
+            select(Post).where(Post.id == post.id).options(selectinload(Post.tags))
+        )
+        loaded = result.scalar_one()
+        assert loaded.tags == []
+
+    @pytest.mark.anyio
+    async def test_ignore_conflicts_true(self, db_session: AsyncSession):
+        """Duplicate inserts are silently skipped when ignore_conflicts=True."""
+        user = User(username="m2m_author4", email="m2m4@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post D", author_id=user.id)
+        tag = Tag(name="duplicate_tag")
+        db_session.add_all([post, tag])
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags, tag)
+
+        # Second call with ignore_conflicts=True must not raise
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags, tag, ignore_conflicts=True)
+
+        result = await db_session.execute(
+            select(Post).where(Post.id == post.id).options(selectinload(Post.tags))
+        )
+        loaded = result.scalar_one()
+        assert len(loaded.tags) == 1
+
+    @pytest.mark.anyio
+    async def test_ignore_conflicts_false_raises(self, db_session: AsyncSession):
+        """Duplicate inserts raise IntegrityError when ignore_conflicts=False (default)."""
+        user = User(username="m2m_author5", email="m2m5@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post E", author_id=user.id)
+        tag = Tag(name="conflict_tag")
+        db_session.add_all([post, tag])
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags, tag)
+
+        with pytest.raises(IntegrityError):
+            async with get_transaction(db_session):
+                await m2m_add(db_session, post, Post.tags, tag)
+
+    @pytest.mark.anyio
+    async def test_non_m2m_raises_type_error(self, db_session: AsyncSession):
+        """Passing a non-M2M relationship attribute raises TypeError."""
+        user = User(username="m2m_author6", email="m2m6@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        role = Role(name="type_err_role")
+        db_session.add(role)
+        await db_session.flush()
+
+        with pytest.raises(TypeError, match="Many-to-Many"):
+            await m2m_add(db_session, user, User.role, role)
+
+    @pytest.mark.anyio
+    async def test_works_inside_lock_tables(self, db_session: AsyncSession):
+        """m2m_add works correctly inside a lock_tables nested transaction."""
+        user = User(username="m2m_lock_author", email="m2m_lock@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        async with lock_tables(db_session, [Tag]):
+            tag = Tag(name="locked_tag")
+            db_session.add(tag)
+            await db_session.flush()
+
+            post = Post(title="Post Lock", author_id=user.id)
+            db_session.add(post)
+            await db_session.flush()
+
+            await m2m_add(db_session, post, Post.tags, tag)
+
+        result = await db_session.execute(
+            select(Post).where(Post.id == post.id).options(selectinload(Post.tags))
+        )
+        loaded = result.scalar_one()
+        assert len(loaded.tags) == 1
+        assert loaded.tags[0].name == "locked_tag"
+
+
+class _LocalBase(DeclarativeBase):
+    pass
+
+
+_comp_assoc = Table(
+    "_comp_assoc",
+    _LocalBase.metadata,
+    Column("owner_id", Uuid, ForeignKey("_comp_owners.id"), primary_key=True),
+    Column("item_group", String(50), primary_key=True),
+    Column("item_code", String(50), primary_key=True),
+    ForeignKeyConstraint(
+        ["item_group", "item_code"],
+        ["_comp_items.group_id", "_comp_items.item_code"],
+    ),
+)
+
+
+class _CompOwner(_LocalBase):
+    __tablename__ = "_comp_owners"
+    id: Mapped[uuid.UUID] = mapped_column(Uuid, primary_key=True, default=uuid.uuid4)
+    items: Mapped[list["_CompItem"]] = relationship(secondary=_comp_assoc)
+
+
+class _CompItem(_LocalBase):
+    __tablename__ = "_comp_items"
+    group_id: Mapped[str] = mapped_column(String(50), primary_key=True)
+    item_code: Mapped[str] = mapped_column(String(50), primary_key=True)
+
+
+class TestM2MRemove:
+    """Tests for m2m_remove helper."""
+
+    async def _setup(
+        self, session: AsyncSession, username: str, email: str, *tag_names: str
+    ):
+        """Create a user, post, and tags; associate all tags with the post."""
+        user = User(username=username, email=email)
+        session.add(user)
+        await session.flush()
+
+        post = Post(title=f"Post {username}", author_id=user.id)
+        tags = [Tag(name=n) for n in tag_names]
+        session.add(post)
+        session.add_all(tags)
+        await session.flush()
+
+        async with get_transaction(session):
+            await m2m_add(session, post, Post.tags, *tags)
+
+        return post, tags
+
+    async def _load_tags(self, session: AsyncSession, post: Post) -> list[Tag]:
+        result = await session.execute(
+            select(Post).where(Post.id == post.id).options(selectinload(Post.tags))
+        )
+        return result.scalar_one().tags
+
+    @pytest.mark.anyio
+    async def test_removes_single(self, db_session: AsyncSession):
+        """Removes one association, leaving others intact."""
+        post, (tag1, tag2) = await self._setup(
+            db_session, "rm_author1", "rm1@test.com", "tag_rm_a", "tag_rm_b"
+        )
+
+        async with get_transaction(db_session):
+            await m2m_remove(db_session, post, Post.tags, tag1)
+
+        remaining = await self._load_tags(db_session, post)
+        assert len(remaining) == 1
+        assert remaining[0].id == tag2.id
+
+    @pytest.mark.anyio
+    async def test_removes_multiple(self, db_session: AsyncSession):
+        """Removes multiple associations in one call."""
+        post, (tag1, tag2, tag3) = await self._setup(
+            db_session, "rm_author2", "rm2@test.com", "tag_rm_c", "tag_rm_d", "tag_rm_e"
+        )
+
+        async with get_transaction(db_session):
+            await m2m_remove(db_session, post, Post.tags, tag1, tag3)
+
+        remaining = await self._load_tags(db_session, post)
+        assert len(remaining) == 1
+        assert remaining[0].id == tag2.id
+
+    @pytest.mark.anyio
+    async def test_noop_for_empty_related(self, db_session: AsyncSession):
+        """Calling with no related instances is a no-op."""
+        post, (tag,) = await self._setup(
+            db_session, "rm_author3", "rm3@test.com", "tag_rm_f"
+        )
+
+        async with get_transaction(db_session):
+            await m2m_remove(db_session, post, Post.tags)
+
+        remaining = await self._load_tags(db_session, post)
+        assert len(remaining) == 1
+
+    @pytest.mark.anyio
+    async def test_idempotent_for_missing_association(self, db_session: AsyncSession):
+        """Removing a non-existent association does not raise."""
+        post, (tag1,) = await self._setup(
+            db_session, "rm_author4", "rm4@test.com", "tag_rm_g"
+        )
+        tag2 = Tag(name="tag_rm_h")
+        db_session.add(tag2)
+        await db_session.flush()
+
+        # tag2 was never associated — should not raise
+        async with get_transaction(db_session):
+            await m2m_remove(db_session, post, Post.tags, tag2)
+
+        remaining = await self._load_tags(db_session, post)
+        assert len(remaining) == 1
+
+    @pytest.mark.anyio
+    async def test_non_m2m_raises_type_error(self, db_session: AsyncSession):
+        """Passing a non-M2M relationship attribute raises TypeError."""
+        user = User(username="rm_author5", email="rm5@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        role = Role(name="rm_type_err_role")
+        db_session.add(role)
+        await db_session.flush()
+
+        with pytest.raises(TypeError, match="Many-to-Many"):
+            await m2m_remove(db_session, user, User.role, role)
+
+    @pytest.mark.anyio
+    async def test_removes_composite_pk_related(self):
+        """Composite-PK branch: DELETE uses tuple IN when related side has multi-col PK."""
+        engine = create_async_engine(DATABASE_URL, echo=False)
+        async with engine.begin() as conn:
+            await conn.run_sync(_LocalBase.metadata.create_all)
+
+        session_factory = async_sessionmaker(engine, expire_on_commit=False)
+        try:
+            async with session_factory() as session:
+                owner = _CompOwner()
+                item1 = _CompItem(group_id="g1", item_code="c1")
+                item2 = _CompItem(group_id="g1", item_code="c2")
+                session.add_all([owner, item1, item2])
+                await session.flush()
+
+                async with get_transaction(session):
+                    await m2m_add(session, owner, _CompOwner.items, item1, item2)
+
+                async with get_transaction(session):
+                    await m2m_remove(session, owner, _CompOwner.items, item1)
+
+                await session.commit()
+
+            async with session_factory() as verify:
+                from sqlalchemy import select
+                from sqlalchemy.orm import selectinload
+
+                result = await verify.execute(
+                    select(_CompOwner)
+                    .where(_CompOwner.id == owner.id)
+                    .options(selectinload(_CompOwner.items))
+                )
+                loaded = result.scalar_one()
+                assert len(loaded.items) == 1
+                assert (loaded.items[0].group_id, loaded.items[0].item_code) == (
+                    "g1",
+                    "c2",
+                )
+        finally:
+            async with engine.begin() as conn:
+                await conn.run_sync(_LocalBase.metadata.drop_all)
+            await engine.dispose()
+
+
+class TestM2MSet:
+    """Tests for m2m_set helper."""
+
+    async def _load_tags(self, session: AsyncSession, post: Post) -> list[Tag]:
+        result = await session.execute(
+            select(Post).where(Post.id == post.id).options(selectinload(Post.tags))
+        )
+        return result.scalar_one().tags
+
+    @pytest.mark.anyio
+    async def test_replaces_existing_set(self, db_session: AsyncSession):
+        """Replaces the full association set atomically."""
+        user = User(username="set_author1", email="set1@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post Set A", author_id=user.id)
+        tag1 = Tag(name="tag_set_a")
+        tag2 = Tag(name="tag_set_b")
+        tag3 = Tag(name="tag_set_c")
+        db_session.add_all([post, tag1, tag2, tag3])
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags, tag1, tag2)
+
+        async with get_transaction(db_session):
+            await m2m_set(db_session, post, Post.tags, tag3)
+
+        remaining = await self._load_tags(db_session, post)
+        assert len(remaining) == 1
+        assert remaining[0].id == tag3.id
+
+    @pytest.mark.anyio
+    async def test_clears_all_when_no_related(self, db_session: AsyncSession):
+        """Passing no related instances clears all associations."""
+        user = User(username="set_author2", email="set2@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post Set B", author_id=user.id)
+        tag = Tag(name="tag_set_d")
+        db_session.add_all([post, tag])
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_add(db_session, post, Post.tags, tag)
+
+        async with get_transaction(db_session):
+            await m2m_set(db_session, post, Post.tags)
+
+        remaining = await self._load_tags(db_session, post)
+        assert remaining == []
+
+    @pytest.mark.anyio
+    async def test_set_on_empty_then_populate(self, db_session: AsyncSession):
+        """m2m_set works on a post with no existing associations."""
+        user = User(username="set_author3", email="set3@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        post = Post(title="Post Set C", author_id=user.id)
+        tag1 = Tag(name="tag_set_e")
+        tag2 = Tag(name="tag_set_f")
+        db_session.add_all([post, tag1, tag2])
+        await db_session.flush()
+
+        async with get_transaction(db_session):
+            await m2m_set(db_session, post, Post.tags, tag1, tag2)
+
+        remaining = await self._load_tags(db_session, post)
+        assert {t.id for t in remaining} == {tag1.id, tag2.id}
+
+    @pytest.mark.anyio
+    async def test_non_m2m_raises_type_error(self, db_session: AsyncSession):
+        """Passing a non-M2M relationship attribute raises TypeError."""
+        user = User(username="set_author4", email="set4@test.com")
+        db_session.add(user)
+        await db_session.flush()
+
+        role = Role(name="set_type_err_role")
+        db_session.add(role)
+        await db_session.flush()
+
+        with pytest.raises(TypeError, match="Many-to-Many"):
+            await m2m_set(db_session, user, User.role, role)

tests/test_pytest.py

@@ -536,11 +536,6 @@ class TestCreateWorkerDatabase:
         await engine.dispose()
 
 
-# ---------------------------------------------------------------------------
-# Local models for composite-PK coverage (own Base → own tables, isolated)
-# ---------------------------------------------------------------------------
-
-
 class _LocalBase(DeclarativeBase):
     pass