mongoose deep populate based on query

I'm populating the mongoose model using deepPopulate plugin like so:

User.findOne({_id: req.params.user_id})
        .deepPopulate('classes.assignments.submissions')
        .exec(function (err, user) {
            res.send(user);
        })

And I want the submissions to be populated based on the query - only submissions for a current user - something like .where(submissions.user == req.params.user_id). I can't find the solution for this and end up in a situation where current user can see submissions from other users.

I was trying to set submissions for other users to null manually inside the exec:

for (var i = 0; i < user.classes.length; i++) {
    var cl = user.classes[i];
    for (var i = 0; i < cl.assignments.length; i++) {
        var assign = cl.assignments[i];
        for (var i = 0; i < assign.submissions.length; i++) {
            var subm = assign.submissions[i];
            if(subm.user != req.params.user_id){
                subm = null
            };
        };
    };
};

But it still returns not filtered submissions.