Quick note: If you are using an ORM, maybe printing the object model will be better.
When I have to read a project without documentation or comment, I do it myself. I assume you can't talk to the former developers.
As I browse the source code and try to understand, I comment the code and update a general architecture document.
Sometimes I'm wrong, so I have to go back and so on. But at the end, commenting and writing general doc is just like taking notes, so it's painless and very useful.
Doing it with one colleague is much more productive.
That particular task can be very painful if you convince yourself it will be painful, which most of us will instinctively do. The idea is to imagine yourself as an explorer rather than the boss servant. The more it's hard to find the secrets, the more fun it is.