NotImplementedError: _db fixture not defined.

I've defined a test fixture _db (based on documentation example) but am getting the following when running pytest:

```NotImplementedError: _db fixture not defined. The pytest-flask-sqlalchemy plugin requires you to define a _db fixture that returns a SQLAlchemy Session with access to your test database. For more information, see the plugin documentation: https://github.com/jeancochrane/pytest-flask-sqlalchemy#conftest-setup```

python version 3.6.9
pytest-flask-sqlalchemy==1.0.2
pytest==5.4.3
pytest-mock==3.2.0
Flask-SQLAlchemy==2.4.4
SQLAlchemy==1.3.18
postgresql v10