fastapi-toolsets/docs/module/fixtures.md

Fixtures

Dependency-aware database seeding with context-based loading strategies.

Overview

The fixtures module lets you define named fixtures with dependencies between them, then load them into the database in the correct order. Fixtures can be scoped to contexts (e.g. base data, testing data) so that only the relevant ones are loaded for each environment.

Defining fixtures

from fastapi_toolsets.fixtures import FixtureRegistry, Context

fixtures = FixtureRegistry()

@fixtures.register
def roles():
    return [
        Role(id=1, name="admin"),
        Role(id=2, name="user"),
    ]

@fixtures.register(depends_on=["roles"], contexts=[Context.TESTING])
def test_users():
    return [
        User(id=1, username="alice", role_id=1),
        User(id=2, username="bob", role_id=2),
    ]

Dependencies declared via depends_on are resolved topologically — roles will always be loaded before test_users.

Loading fixtures

By context with load_fixtures_by_context:

from fastapi_toolsets.fixtures import load_fixtures_by_context

async with db_context() as session:
    await load_fixtures_by_context(session, fixtures, Context.TESTING)

Directly by name with load_fixtures:

from fastapi_toolsets.fixtures import load_fixtures

async with db_context() as session:
    await load_fixtures(session, fixtures, "roles", "test_users")

Both functions return a dict[str, list[...]] mapping each fixture name to the list of loaded instances.

Contexts

Context is an enum with predefined values:

Context	Description
Context.BASE	Core data required in all environments
Context.TESTING	Data only loaded during tests
Context.DEVELOPMENT	Data only loaded in development
Context.PRODUCTION	Data only loaded in production

A fixture with no contexts defined takes Context.BASE by default.

Custom contexts

Plain strings and any Enum subclass are accepted wherever a Context enum is expected.

from enum import Enum

class AppContext(str, Enum):
    STAGING = "staging"
    DEMO = "demo"

@fixtures.register(contexts=[AppContext.STAGING])
def staging_data():
    return [Config(key="feature_x", enabled=True)]

await load_fixtures_by_context(session, fixtures, AppContext.STAGING)

Default context for a registry

Pass contexts to FixtureRegistry to set a default for all fixtures registered in it:

testing_registry = FixtureRegistry(contexts=[Context.TESTING])

@testing_registry.register          # implicitly contexts=[Context.TESTING]
def test_orders():
    return [Order(id=1, total=99)]

Same fixture name, multiple context variants

The same fixture name may be registered under different (non-overlapping) context sets. When multiple contexts are loaded together, all matching variants are merged:

@fixtures.register(contexts=[Context.BASE])
def users():
    return [User(id=1, username="admin")]

@fixtures.register(contexts=[Context.TESTING])
def users():
    return [User(id=2, username="tester")]

# loads both admin and tester
await load_fixtures_by_context(session, fixtures, Context.BASE, Context.TESTING)

Registering two variants with overlapping context sets raises ValueError.

Load strategies

LoadStrategy controls how the fixture loader handles rows that already exist:

Strategy	Description
LoadStrategy.INSERT	Insert only, fail on duplicates
LoadStrategy.MERGE	Insert or update on conflict (default)
LoadStrategy.SKIP_EXISTING	Skip rows that already exist

await load_fixtures_by_context(
    session, fixtures, Context.BASE, strategy=LoadStrategy.SKIP_EXISTING
)

Merging registries

Split fixture definitions across modules and merge them:

from myapp.fixtures.dev import dev_fixtures
from myapp.fixtures.prod import prod_fixtures

fixtures = FixtureRegistry()
fixtures.include_registry(registry=dev_fixtures)
fixtures.include_registry(registry=prod_fixtures)

Fixtures with the same name are allowed as long as their context sets do not overlap. Conflicting contexts raise ValueError.

Looking up fixture instances

get_obj_by_attr retrieves a specific instance from a fixture function by attribute value — useful when building cross-fixture depends_on relationships:

from fastapi_toolsets.fixtures import get_obj_by_attr

@fixtures.register(depends_on=["roles"])
def users():
    admin_role = get_obj_by_attr(roles, "name", "admin")
    return [User(id=1, username="alice", role_id=admin_role.id)]

Raises StopIteration if no matching instance is found.

Pytest integration

Use register_fixtures to expose each fixture in your registry as an injectable pytest fixture named fixture_{name} by default:

# conftest.py
import pytest
from fastapi_toolsets.pytest import create_db_session, register_fixtures
from app.fixtures import registry
from app.models import Base

DATABASE_URL = "postgresql+asyncpg://user:pass@localhost/test_db"

@pytest.fixture
async def db_session():
    async with create_db_session(database_url=DATABASE_URL, base=Base, cleanup=True) as session:
        yield session

register_fixtures(registry=registry, namespace=globals())

# test_users.py
async def test_user_can_login(fixture_users: list[User], fixture_roles: list[Role]):
    ...

The load order is resolved automatically from the depends_on declarations in your registry. Each generated fixture receives db_session as a dependency and returns the list of loaded model instances.

CLI integration

Fixtures can be triggered from the CLI. See the CLI module for setup instructions.

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