Code for http://alexmic.net/flask-sqlalchemy-pytest/

conftest.py

import os
import pytest

from alembic.command import upgrade
from alembic.config import Config

from project.factory import create_app
from project.database import db as _db


TESTDB = 'test_project.db'
TESTDB_PATH = "/opt/project/data/{}".format(TESTDB)
TEST_DATABASE_URI = 'sqlite:///' + TESTDB_PATH


ALEMBIC_CONFIG = '/opt/project/alembic.ini'


@pytest.fixture(scope='session')
def app(request):
    """Session-wide test `Flask` application."""
    settings_override = {
        'TESTING': True,
        'SQLALCHEMY_DATABASE_URI': TEST_DATABASE_URI
    }
    app = create_app(__name__, settings_override)

    # Establish an application context before running the tests.
    ctx = app.app_context()
    ctx.push()

    def teardown():
        ctx.pop()

    request.addfinalizer(teardown)
    return app


def apply_migrations():
    """Applies all alembic migrations."""
    config = Config(ALEMBIC_CONFIG)
    upgrade(config, 'head')


@pytest.fixture(scope='session')
def db(app, request):
    """Session-wide test database."""
    if os.path.exists(TESTDB_PATH):
        os.unlink(TESTDB_PATH)

    def teardown():
        _db.drop_all()
        os.unlink(TESTDB_PATH)

    _db.app = app
    apply_migrations()

    request.addfinalizer(teardown)
    return _db


@pytest.fixture(scope='function')
def session(db, request):
    """Creates a new database session for a test."""
    connection = db.engine.connect()
    transaction = connection.begin()

    options = dict(bind=connection, binds={})
    session = db.create_scoped_session(options=options)
    
    db.session = session

    def teardown():
        transaction.rollback()
        connection.close()
        session.remove()

    request.addfinalizer(teardown)
    return session

database.py

from flask.ext.sqlalchemy import SQLAlchemy, SignallingSession, SessionBase


class _SignallingSession(SignallingSession):
    """A subclass of `SignallingSession` that allows for `binds` to be specified
    in the `options` keyword arguments.

    """
    def __init__(self, db, autocommit=False, autoflush=True, **options):
        self.app = db.get_app()
        self._model_changes = {}
        self.emit_modification_signals = \
            self.app.config['SQLALCHEMY_TRACK_MODIFICATIONS']

        bind = options.pop('bind', None)
        if bind is None:
            bind = db.engine

        binds = options.pop('binds', None)
        if binds is None:
            binds = db.get_binds(self.app)

        SessionBase.__init__(self,
                             autocommit=autocommit,
                             autoflush=autoflush,
                             bind=bind,
                             binds=binds,
                             **options)


class _SQLAlchemy(SQLAlchemy):
    """A subclass of `SQLAlchemy` that uses `_SignallingSession`."""
    def create_session(self, options):
        return _SignallingSession(self, **options)


db = _SQLAlchemy()

Looks like signalling session from flask-sqlalchemy should work out of the box now mitsuhiko/flask-sqlalchemy@ac135c6

Thank you to share this example code! This is really helpful.
Would it be possible to share also an example of the project.factory module?

I got the following error when I was trying this example (excluding the custom DB file):
TypeError: __init__() got multiple values for keyword argument 'binds'
What fixed it for me is removing binds on Line 68

I'm having the same issue as @krmarien - the example won't work without that factory function!

Hi @krmarien and @flyte in case you're still searching, my solution was to substitute app = create_app(__name__, settings_override) with:

app = Flask(__name__)
app.config.update(settings_override)

Which is what, I assume, the function must do, given it's name and arguments.

And of course, the change @vlttnv mentioned is also required, since the gist were probably written with an older version of flask-sqlalchemy in mind, i.e pre- mitsuhiko/flask-sqlalchemy@ac135c6 as @chekunkov mentions. After these changes, this recipe works like a charm. Thanks @alexmic !

There I did it, I mentioned everyone in the thread 🎉 !

Bai
@asfaltboy

I have code that imports the session from a models.py directly. Overwriting that (models.session = ourNewSession) does not work since it's already imported. However, since it's a ScopedSession, we can trick it:

def session(db, request):
    ...
    session = db.create_scoped_session(options={'bind': connection, 'binds': {}})
    def getSession():
        return session
    models.session.registry = getSession
    ...

Is it possible to change the app to used the session with rollback too? It doesn't look like this will work with app.test_client() for example.

@chekunkov, the commit you pointed to didn't actually help: empty dict {} is still recognized as False and is replaced with db.get_binds(app) anyway. The only actual change that commit introduced is that db.get_binds() function is not called if binds argument passed and is not-False.
I tried to pass "mock" value like {None: None} which would not be recognized as False, but it didn't help and instead caused an error.
So there is still a problem in flask-sqlalchemy SignallingSession contructor. Will try to make a pull request.

Hm, they already fixed it in upstream, but current version 2.1 still didn't receive that fix.
Here is my approach for work-around:

from flask_sqlalchemy import SignallingSession

def wrap_signalling_session(cls):
    _original_init = cls.__init__
    def _fixed_init(self, *args, **kwargs):
        binds = kwargs.get('binds')
        if binds == {}:
            def empty(*args, **kwargs):
                pass
            self._add_bind = empty  # override class' method with a function
        _original_init(self, *args, **kwargs)
        if binds == {}:
            del self._add_bind
    cls.__init__ = _fixed_init
wrap_signalling_session(SignallingSession)

Another catch: the approach described in the article is great, but it has a non-obvious caveat - took me several hours to debug.
Here is the problem: if your code always uses db.session, everything will work fine. But the famous Flask-Admin extension expects you to pass a session object when you instantiate a flask_admin.contrib.sqla.ModelView. As a result, it will use original session (the scoped_session object which was created during app initialization), not test-specific one.
I worked it around using LocalProxy to access current session:

from werkzeug.local import LocalProxy

db = ...

def init_app():
    session = LocalProxy(lambda: db.session)
    admin = ...
    admin.add_view(ModelView(User, session))

As a result, it always uses the correct session.

I didn't want to have to change my application code to fix my tests, so I opted for the hack suggested here and faithfully reproduced for your convenience:

class _dict(dict):
    def __nonzero__(self):
        return True

connection = db.engine.connect()
transaction = connection.begin()
options = dict(bind=connection, binds=_dict())
session = db.create_scoped_session(options=options)

Thanks for the help y'all!

Could somebody please explain to me why the line _db.app = app is needed in the db fixture?

@stefanprobst, it seems like he is doing that to replace the live app instance with the test app instance which he creates with the app fixture and which has specific test related config values set in settings_override (namely TESTING and SQLALCHEMY_DATABASE_URI).
So without that line the _SQLAlchemy instance wouldn't have the proper config values set for the test environment.

I was using Flask 0.11 and Line 76: session.remove() wasnt working, updated to Flask==0.12(latest as for now) and worked.
(Probably this Flask PR has to do with the fix.)
(spent some hours until i realize why it didn't work in case anybody else wastes time on that)