I did the oppisite, I needed auth in a toy application I was writing and was dreading writing and getting correct the whole auth layer, so I thought to myself "self, postgres has a very rich and through auth system, why not just use that" So I did just that, I just pass the user credentials straight through to to the database connection and let the database decide what access the user gets to the data.

I am not sure if I would do this on something serious, but it was a lot of fun to set up and saved me a lot of code by reusing the comprehensive auth system that lives with the actual data in question.