Overcoming Data Loss and Test Flakiness: Solutions for Using SQLite In-Memory Database with NHibernate

Example:

Imagine you have a unit test that interacts with a database using NHibernate and SQLite ":memory:".

public class MyTest
{
    [Test]
    public void TestDataPersistence()
    {
        // Open session 1
        ISession session1 = sessionFactory.OpenSession();

        // Save some data
        MyEntity entity = new MyEntity { Name = "Test Entity" };
        session1.Save(entity);

        // Close session 1

        // Open session 2 (new connection)
        ISession session2 = sessionFactory.OpenSession();

        // Try to retrieve data
        MyEntity retrievedEntity = session2.Get<MyEntity>(entity.Id);

        // This will likely be null, as session2 is using a different in-memory database

        // Close session 2
    }
}

In this example, session1 saves an entity to the in-memory database. However, when session2 opens a new connection, it creates a separate in-memory database, so it cannot find the previously saved data.

Related Issues:

• Data loss: As mentioned above, any data saved in the in-memory database gets lost when the connection closes.
• Test flakiness: In unit tests, this can lead to inconsistent results depending on the order of test execution.
• "No such table" errors: If you try to access tables created in a previous session, you might encounter "no such table" errors since the tables don't exist in the newly created in-memory database.

Here are two solutions to address this issue:

Reusing the same connection:

• Open a single session at the beginning of your test suite (e.g., in a [TestFixtureSetUp]) method.
• Use the connection from that session for all subsequent operations in your tests.

This ensures that all tests use the same in-memory database, maintaining data persistence throughout the test run.

Using a dedicated connection provider:

• Implement a custom connection provider that manages a single connection to the in-memory database.
• Configure NHibernate to use this provider instead of the default connection pool.
• This approach offers more control over the connection lifecycle, but it requires more code implementation.

Additional Considerations:

• While these solutions address the issue for testing scenarios, using in-memory databases for production applications is not recommended due to data persistence concerns.
• For production scenarios, consider using a persistent database like SQLite on disk or other databases like PostgreSQL or MySQL.