Unit-tests for a working codebase with limited time: How?

Question

I have a few medium-sized Rails apps that I work on routinely, and only one of them has any unit tests at all. But I have seen the light and I want to change all that, except... I don't have the time to go in and starting writing tests class by class or something like that.

How do you start writing unit tests on an existing -- and working -- codebase with limited time? For example, since any approach would have to be incremental, how would you order your unit-test writing? Start with superficial tests, then move on to more coverage, or cover just a few classes... etc.

Note: I am asking this question thinking about Rails, but really I'm interested in how it applies to any language.

Edit: Note, this question is not the same as this other one. The other one asks how hard this is, and was the result worth it. I'm asking about how to add unit tests.

Answer 1

Here is how I usually start adding unit tests to a project that didn't start out that way: Wait for someone to file a bug, then write a unit test that reproduces the bug. Then fix the unit test. This not only starts building unit tests, but now no one can accuse you of a regression for the given bug.