bookshelf.js many to many relationship not working (belongsToMany)

I can't get to work a many-to-many relationship using bookshelf.js.

Models:

var Event = bookshelf.Model.extend({
    tableName: 'events',
    idAttribute: 'event_id',
    tags: function() {
        return this.belongsToMany(Tag, 'events_tags', 'event_id', 'tag_id');
    }
});

var Tag = bookshelf.Model.extend({
   tableName: 'tags',
   idAttribute: 'tag_id',
   events: function() {
      return this.belongsToMany(Event, 'events_tags', 'tag_id', 'event_id');
   }
});

Database table "events":

  event_id      name
 -------------------------------------------------
      1          party at my place

Database table "tags":

  tag_id         name
 ---------------------------
      1           music

Database table "events_tags":

  event_tag_id          event_id         tag_id
 -------------------------------------------------------------
      1                   1                 1

Query that isn't returning what it should:

new db.Event()
.fetchAll({withRelated: ['tags']})
.then(function(events) {

    console.log(events.toJSON());
});

The log outputs:

 [{'event_id': 1, 'name': 'party at my place', 'tags': []}]

That is: empty array of tags, which is wrong.

Question: Is there an issue with my code? the desired output should be:

 [{
   'event_id': 1, 'name': 'party at my place', 
   'tags': [{'tagId':1, 'name':'music'}]
 }]

This doesn't help in the question, but I'll throw it anyway: I'm currently hating all ORMs in Node JS! Maybe I should just use a query builder.