Make an attribute required based on param with Sails.js

I have a model with two attributes. One called userType and one called lastName.

Basically what I want to do is: if userType is professor then lastName should be required.

userType: {
  type: 'string',
  required: true,
  enum: ['professor', 'administrator']
},

lastName: {
  type: 'string',
  required: function() {
    return this.userType === 'professor';
  }
},

Why can't I pass required as a function? In the waterline documentation there is even an example with the validation contains.

If not possible, does it have any other way to do this? I don't want to create custom validations on the controller, I want to leave everything to the model. Maybe even use the beforeValidate callback.

Thanks