Items created in `db_session` not accessible with `client_class`

I have a conftest:
```python
import os

import pytest
from flask_migrate import upgrade

from myapp import create_app, flask_app

@pytest.fixture(scope='session')
def database(request):
uri = flask_app.config.get('SQLALCHEMY_DATABASE_URI')
assert uri.startswith('sqlite:///'), 'In tests, use a Sqlite DB'
sqlite_path = uri.replace('sqlite:///', '')
if os.path.exists(sqlite_path):
os.remove(sqlite_path)

@request.addfinalizer
def drop_database():
os.remove(sqlite_path)

@pytest.fixture(scope='session')
def app(request, database):
"""
Used by pytest-flask
"""
app = create_app()
with app.app_context():
upgrade()
return app

@pytest.fixture(scope='session')
def _db(request, app):
"""
Used by pytest-flask-sqlalchemy
"""
db = app.extensions['sqlalchemy'].db
return db
```

And then have:

```python
import pytest
from mymodels import MyItem
from flask import url_for

@pytest.mark.usefixtures('client_class')
class TestMyApp:
def test_delete(self, db_session, uid):
item = MyItem(
name='x'
)
db_session.add(item)
db_session.commit()
res = self.client.delete(url_for('myapi.ItemResource', id=item.id))
assert res.status_code == 200
```

When i run the test, it always fails with a 404 error saying that the provided ID does not exist