Let's start our unit testing journey with the data models we wrote for the Notes application. Because this is unit testing, the models should be tested separately from the rest of the Notes application.
In the case of most of the Notes models, isolating their dependencies implies creating a mock database. Are you going to test the data model or the underlying database? Mocking out a database means creating a fake database implementation, which does not look like a productive use of our time. You can argue that testing a data model is really about testing the interaction between your code and the database, that mocking out the database means not testing that interaction, and therefore we should test our code against the database engine used in production.
With that line of reasoning in mind, we'll skip mocking out the database, and instead run the tests against a database containing test data. To simplify launching the test database, we'll use Docker to start and stop...