How to use PostgreSQL test database in async FastAPI tests?

I'm working on an async FastAPI project and I want to connect to the database during tests. Coming from Django, my instinct was to create pytest fixtures that take care of creating/dropping the test database. However, I couldn't find much documentation on how to do this. The most complete instructions I could find were in this tutorial, but they don't work for me because they are all synchronous. I'm somewhat new to async development so I'm having trouble adapting the code to work async. This is what I have so far:

import pytest
from sqlalchemy.ext.asyncio import create_async_engine, session
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy_utils import database_exists, create_database

from fastapi.testclient import TestClient

from app.core.db import get_session
from app.main import app


Base = declarative_base()


@pytest.fixture(scope="session")
def db_engine():
    default_db = (
        "postgresql+asyncpg://postgres:postgres@postgres:5432/postgres"
    )
    test_db = "postgresql+asyncpg://postgres:postgres@postgres:5432/test"
    engine = create_async_engine(default_db)
    if not database_exists(test_db):  # <- Getting error on this line
        create_database(test_db)

    Base.metadata.create_all(bind=engine)
    yield engine


@pytest.fixture(scope="function")
def db(db_engine):
    connection = db_engine.connect()

    # begin a non-ORM transaction
    connection.begin()

    # bind an individual Session to the connection
    Session = sessionmaker(bind=connection)
    db = Session()
    # db = Session(db_engine)

    yield db

    db.rollback()
    connection.close()


@pytest.fixture(scope="function")
def client(db):
    app.dependency_overrides[get_session] = lambda: db

    PREFIX = "/api/v1/my-endpoint"
    with TestClient(PREFIX, app) as c:
        yield c

And this is the error I'm getting:

E           sqlalchemy.exc.MissingGreenlet: greenlet_spawn has not been called; can't call await_() here. Was IO attempted in an unexpected place? (Background on this error at: https://sqlalche.me/e/14/xd2s)

/usr/local/lib/python3.9/site-packages/sqlalchemy/util/_concurrency_py3k.py:67: MissingGreenlet

Any idea what I have to do to fix it?