title: How can I find another Mongoose document (by ID) from a document's instance method?
body: <p>My model saves the ID of its "parent" (in a tree structure). In one of its instance methods, I want to edit the parent. Is there a way I can look it up via this ID? <code>Model.findById</code> doesn't work, obviously. Do I have access to this function in some other way?</p>