you're really testing your mock here.

What we need to know is that `filterBy` actually called your mock. So in your mock you only need to keep a state variable like `mFindCalledArgs` where you store the actual args received by the `find` method, and that's what you need to test in your test;

Also, you'll need an asynchronous test here (and in the other tests too). This is done with a `done` argument to the function, and calling `done()` at the end of the callback.