Every Test Gets a Fresh Postgres Database: Fast Per-Test Isolation in SQLAlchemy

Fast per-test isolation in SQLAlchemy requires each test to run against its own Postgres database, cloned from a template to maintain speed, but a key Postgres rule turned this setup into a 20-minute CI outage. The method involves creating a single template database upfront, from which individual test databases are cloned, allowing for real commit testing and preventing leftover data between tests. This approach bypasses the need for repeated schema setups, saving time. However, a critical issue arose when tests that manipulated connection lifetimes left connections open on the template, causing cloning attempts to fail intermittently and silently. The solution was to ensure the clone path evicts any existing connections from the template, aligning with the teardown process, thereby maintaining test isolation and CI stability. This method, while beneficial for its speed and accuracy in mimicking production environments, requires careful management of database connections and is a responsibility of the developer when using unopinionated tools like SQLAlchemy.

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