guys, a lame question from a newbie who failed to google it. let's say i have a schema and in the schema, i want to perform a custom validation against something in the database. but everywhere i look, it's said that to do mongo operations i need the mongoose model. i can't call a model in a schema, because the model doesn't exist yet, and i fail to find a way to create the model first and then apply the schema to it. how do you guys deal with that?