Trying to remove a milestone and then the reference to the milestone from an event

stupidmilestone.js

app.post('/milestone/:action', function(req, res) {

    // Get the action
    if( req.params.action == 'delete' ) {

        // Get the POST variables
        var milestone_id = req.body.milestone_id;
        var event_id = req.body.event_id;

        // We need to delete the milestone
        MilestoneModel.remove({_id: milestone_id }, function(err, m) {

            if( ! err ) {
              // THis next line always returns 'undefined' for event_id
                console.log('trying to find event: ' + event_id);
                
                EventModel.findOne({ _id: event_id }, function(err, event_object) {
                    
                    if(event_object) {
                        event_object.update( { field: 'milestones' }, { $pull: { field: milestone_id }}, function(err, event ) {
                            
                            if( ! err ) {
                                res.json({success: true});
                            } else {
                                console.log('Could not remove milestone from event: ' + event_id);
                                res.json({success: true, remove_event: false});
                            }
                        });
                    } else {
                        res.json({ success: false });
                    }

                });

            } else {

                res.json({success: false, error: err});
            }
            
        });
    }

});