I wouldn’t mock a data layer unless I was unit testing (for example) response codes on a route depending on the data layer. In those cases you aren’t interested

I wouldn’t mock a data layer unless I was unit testing (for example) response codes on a route depending on the data layer. In those cases you aren’t interested in the data as much as how it is handled. I like to be able to point an integration test at a “clean” database in order to test a data layer. In this way you can have separate tests to basically ensure your API responses and database queries are all valid given your database is in a valid state and the API request is valid. With this kind of setup I rarely actually have to run an application anymore to feel it is in good working condition, and that saves me a LOT of time.