How do I write SQLAlchemy test fixtures for FastAPI applications - sqlalchemy

Question

I am writing a FastAPI application that uses a SQLAlchemy database. I have copied the example from the FastAPI documentation, simplifying the database schema for concisions' sake. The complete source is at the bottom of this post.
This works. I can run it with uvicorn sql_app.main:app and interact with the database via the Swagger docs. When it runs it creates a test.db in the working directory.
Now I want to add a unit test. Something like this.
from fastapi import status
from fastapi.testclient import TestClient
from pytest import fixture
from main import app
#fixture
def client() -> TestClient:
return TestClient(app)
def test_fast_sql(client: TestClient):
response = client.get("/users/")
assert response.status_code == status.HTTP_200_OK
assert response.json() == []
Using the source code below, this takes the test.db in the working directory as the database. Instead I want to create a new database for every unit test that is deleted at the end of the test.
I could put the global database.engine and database.SessionLocal inside an object that is created at runtime, like so:
class UserDatabase:
def __init__(self, directory: Path):
directory.mkdir(exist_ok=True, parents=True)
sqlalchemy_database_url = f"sqlite:///{directory}/store.db"
self.engine = create_engine(
sqlalchemy_database_url, connect_args={"check_same_thread": False}
)
self.SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=self.engine)
models.Base.metadata.create_all(bind=self.engine)
but I don't know how to make that work with main.get_db, since the Depends(get_db) logic ultimately assumes database.engine and database.SessionLocal are available globally.
I'm used to working with Flask, whose unit testing facilities handle all this for you. I don't know how to write it myself. Can someone show me the minimal changes I'd have to make in order to generate a new database for each unit test in this framework?
The complete source of the simplified FastAPI/SQLAlchemy app is as follows.
database.py
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
SQLALCHEMY_DATABASE_URL = "sqlite:///./test.db"
engine = create_engine(
SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}
)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base = declarative_base()
models.py
from sqlalchemy import Column, Integer, String
from database import Base
class User(Base):
__tablename__ = "users"
id = Column(Integer, primary_key=True, index=True)
name = Column(String)
age = Column(Integer)
schemas.py
from pydantic import BaseModel
class UserBase(BaseModel):
name: str
age: int
class UserCreate(UserBase):
pass
class User(UserBase):
id: int
class Config:
orm_mode = True
crud.py
from sqlalchemy.orm import Session
import schemas
import models
def get_user(db: Session, user_id: int):
return db.query(models.User).filter(models.User.id == user_id).first()
def get_users(db: Session, skip: int = 0, limit: int = 100):
return db.query(models.User).offset(skip).limit(limit).all()
def create_user(db: Session, user: schemas.UserCreate):
db_user = models.User(name=user.name, age=user.age)
db.add(db_user)
db.commit()
db.refresh(db_user)
return db_user
main.py
from typing import List
from fastapi import Depends, FastAPI, HTTPException
from sqlalchemy.orm import Session
import schemas
import models
import crud
from database import SessionLocal, engine
models.Base.metadata.create_all(bind=engine)
app = FastAPI()
# Dependency
def get_db():
try:
db = SessionLocal()
yield db
finally:
db.close()
#app.post("/users/", response_model=schemas.User)
def create_user(user: schemas.UserCreate, db: Session = Depends(get_db)):
return crud.create_user(db=db, user=user)
#app.get("/users/", response_model=List[schemas.User])
def read_users(skip: int = 0, limit: int = 100, db: Session = Depends(get_db)):
users = crud.get_users(db, skip=skip, limit=limit)
return users
#app.get("/users/{user_id}", response_model=schemas.User)
def read_user(user_id: int, db: Session = Depends(get_db)):
db_user = crud.get_user(db, user_id=user_id)
if db_user is None:
raise HTTPException(status_code=404, detail="User not found")
return db_user

Answer 1

You need to override your get_db dependency in your tests, see these docs.
Something like this for your fixture:
#fixture
def db_fixture() -> Session:
raise NotImplementError() # Make this return your temporary session
#fixture
def client(db_fixture) -> TestClient:
def _get_db_override():
return db_fixture
app.dependency_overrides[get_db] = _get_db_override
return TestClient(app)