Prevent going below 0 (zero) into the negatives mongodb

returnsDown: function(studentId) {
    var user = Meteor.user();
    if (!user)
        throw new Meteor.Error(401, "You need to login to upvote");
    var student = Students.findOne(studentId);
    if (!student)
        throw new Meteor.Error(422, 'Student not found');
    Students.update(student._id, {
        $inc: {
            returns: -1

Now this decreases the returns field by 1. Fine. Is there anything mongodb built in that can prevent the going below 0? In this case, when a field value hits 0, no reduction allowed anymore?

Otherwise, the way I’m thinking of is to check if the field is not 0 before decreasing everything, unless there’s a better way.

simple schema does not let you specify it ?

I’m using a schema from aldeed and haven’t noticed any of that sort yet.