In the tutorial events there is this example:

const User = bookshelf.model('User', {
  tableName: 'users',

  initialize() {
    this.on('saving', (model) => {
      return User.forge({email: model.get('email')}).fetch().then((user) => {
        if (user) 
            throw new Error('That email address already exists')
    

When both options are passed, shallow: true should take priority since it is a standard Bookshelf option. Furthermore it should cause all ensureRelationsLoaded behavior to be avoided, since there's no reason to do unnecessary work loading relations that aren't going to be serialized--unless plugin option ensureRelationsVisibleAndInvisible is passed.