Wednesday, January 1, 2014

Unit testing SQLAlchemy apps, part 2: the universal method

The problem

In an earlier post I presented a method for unit testing SQLAlchemy applications cleanly, without leaving any trace of the tables created during the run of the test suite. However, the problem with this method was that it required the target RDBMS to support both transactional DDL and nested transactions. This requirement left two prominent vendors in the cold: MySQL and Oracle.

What didn't work

In search for a solution, I was reminded of the existence of a feature called temporary tables. This seemed like a perfect fit for the problem! Most RDBMS's can create both "local" and "global" temporary tables. Local temporary tables are only visible to the connection that created them, while global temporary tables are structurally visible to all connections, but the data still remains specific to each connection. Unfortunately, Oracle only supports global temporary tables which are unusable for unit testing since they stick around after the test suite has finished. MySQL, however, does support local temporary tables, so I modified the original unit testing code to create all the tables as temporary. But that didn't work either because apparently foreign keys between temporary tables aren't allowed.

Since temporary tables didn't provide a working solution, I had to look into other alternatives. The most obvious one would of course be to reflect the metadata at the beginning and use metadata.drop_all(). This approach, however, has one subtle issue circular relationships. If the tables are linked in a cyclic relationship, like A → B → C → A, then these tables can't be dropped without first dropping the foreign key constraints. But then I heard of the DROP TABLE ... CASCADE command which supposedly drops the foreign keys too along the way. This got my hopes up, even though SQLAlchemy didn't support this directly. Those hopes, however, died quickly when I looked at the MySQL documentation and saw this:
RESTRICT and CASCADE are permitted to make porting easier. In MySQL 5.6, they do nothing.

The solution

With this, I had to admit defeat and settle for the lowest common denominator. As such, the revised testing process goes as follows:
  1. Reflect the metadata from the database reserved for unit testing
  2. Drop all foreign key constraints using the old metadata
  3. Drop all tables using the reflected metadata
  4. Create all tables from the current model
  5. Add any base data (fixtures) and commit the session
  6. Prevent your application and framework from committing or closing the session
  7. Run the tests, rolling back the transaction at the end of each test
Step 1 is necessary because there's no telling how much your model has changed between test runs, so simply running drop_all() with your current model's metadata is not guaranteed to do the right thing.
Step 2 is necessary because of potential circular foreign key dependencies preventing some tables from being dropped (see the previous section for a deeper explanation).
Step 6 is necessary for two reasons:
  1. Allowing commit() would break test isolation by leaking database changes to other tests
  2. Allowing the session to close would mean that any changes made between requests within a single test would be rolled back
Finally, a couple reminders:
  • Remember to point the connection URI to a dedicated testing database so you don't lose development (not to mention production) data
  • When testing on MySQL, use InnoDB tables since MyISAM doesn't support transactions

Putting it to practice with Flask and py.test

I developed this testing method to run unit tests on a new, mid-sized Flask based web app of mine that had to use MySQL as its data store for compatibility with an older version. I've also recently migrated from nose to the wonderful py.test testing framework.

The following code is a generic testing example, adapted from my application's test suite. It works with a single-db configuration, but could be adapted to a multi-db configuration. I've tested it against MySQL 5.5, PostgreSQL 9.1 and SQLite 2.6.0.

It should be mentioned that I experimented with applying the faster method (based on nested transactions) but my test suite ran only 0.6 seconds faster with it and the approach required a whole different code path on several fixtures so I decided to drop it.

from sqlalchemy.schema import MetaData, DropConstraint
import pytest

from yourapp import db, create_app

def app(request):
    return create_app()

@pytest.fixture(scope='session', autouse=True)
def setup_db(request, app):
    # Clear out any existing tables
    metadata = MetaData(db.engine)
    for table in metadata.tables.values():
        for fk in table.foreign_keys:

    # Create the tables based on the current model

    # Add base data here
    # ...

def dbsession(request, monkeypatch):
    # Roll back at the end of every test

    # Prevent the session from closing (make it a no-op) and
    # committing (redirect to flush() instead)
    monkeypatch.setattr(db.session, 'commit', db.session.flush)
    monkeypatch.setattr(db.session, 'remove', lambda: None)

def test_example(app):
    with app.test_client() as client:
        response = client.get('/some/path')
    assert response.status_code == 200
    assert db.session.query(Foo).count() == 5

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