reference data disappearing after pytest run?

[ccurvey]

I'm sure this is something I've done; I'm hoping the community can help me figure out where I went wrong.

I'm using pytest with Django. In my migrations, I create a Category class, and populate it with reference data. I have --reuse-db in my pytest.ini.

A few days ago, some of my tests started failing because some of my Category reference data was missing. I can get my tests to pass if I specify --create-db on the command line. However, after a test run, the Category table exists in Postgres, but it's empty.

Anyone seen something like this before? Any ideas on how I might be able to track down the issue?

[portyu9]

Hi @ccurvey

The key detail here is:

--reuse-db

This is pytest-django behavior rather than pytest core.

--reuse-db deliberately keeps the test database between runs for faster startup. It does not mean that database contents should be treated as persistent application/reference state between test runs.

If you changed migrations or migration-created data, the existing reused test database can be stale. In that situation use:

pytest --reuse-db --create-db

--create-db forces pytest-django to rebuild the test database, after which subsequent runs can reuse it again.

That explains why:

pytest --create-db

makes the tests pass.

There is a second issue to consider, though: tests using Django's database support perform database isolation/cleanup. Data that a test modifies or creates should not be expected to remain in the reused database after the suite.

So I would not rely on:

migration inserts Category rows once
        ↓
reused test DB permanently keeps them
        ↓
all future pytest runs consume them

as your test-data strategy.

If those categories are required for a test, establish that precondition deterministically. Depending on what the data represents, that can be:

• a data migration when the database is created,
• a pytest fixture that creates the required objects,
• or pytest-django's database setup customization for global test-database initialization.

For example, for test-specific reference data:

@pytest.fixture
def categories(db):
    return [
        Category.objects.create(name="A"),
        Category.objects.create(name="B"),
    ]

Then tests that require those categories request the fixture explicitly.

For your immediate problem I would do two things:

1. After any migration/reference-data change, rebuild once:

pytest --reuse-db --create-db

2. Inspect the tests for anything deleting/flushing Category rows, because an empty table after the suite means some test/database cleanup path has removed the records.

So --reuse-db is useful for keeping the expensive database/schema around, but it should not be treated as a mechanism for persisting mutable test data indefinitely.

Please mark this answer as accepted if it helped, thank you.