How to add pre-hook in keystonejs? - keystonejs

Question

I want to add multiple select options field. But the docs state that doesn't allow for multiple select. But recommends pre-hook for that case.
Stores a String or Number in the model. Displayed as a select field in
the Admin UI. Does not allow for multiple items to be selected. If you
want to provide multiple values, you can use TextArray or NumberArray,
although neither will have the same constrained input. You can limit
the options using a pre-save hook.
I search for pre-hook but it seems came from mongoose. And in my case, I create the model using Keystone so that I can use it in admin page
var keystone = require('keystone');
var Types = keystone.Field.Types;
var MyModel = new keystone.List('MyModel');
MyModel.add({
aField: { type: Types.TextArray, required: false, initial: true },
});
so how do I create the pre-hook? for example, I want to limit the TextArray to be set of ('a','b','c')?

Answer 1

I have set up pre-save hooks like this (or something similar to this. Did not test this code).
var keystone = require('keystone');
var Types = keystone.Field.Types;
/**
* Musician Model
* ==========
*/
var Musician = new keystone.List('Musician', {
map: { name: 'title' },
autokey: { path: 'slug', from: 'title', unique: true },
});
Musician.add({
title: { type: String, required: true },
published: { type: Types.Boolean, default: false },
musicianId: { type: String, note: noteUpdateId },
});
Musician.schema.pre('save', function (next) {
console.log(this.title);
console.log(this.isNew);
if (this.isNew) {
// generates a random ID when the item is created
this.musicianId = Math.random().toString(36).slice(-8);
}
next();
});
Musician.defaultColumns = 'title, published, musicianId';
Musician.register();