Ensure empty array for a manyOf datatype

[tarjei]

Hi, I got an entity with

{
  module: {
    lessons: manyOf('lesson')
  }
}

How should I write this so that lessons are always an array i.e never undefined?

[kettanaito]

Hey, @tarjei. Thanks for asking this.

Your model definition seems correct. Here's a recap below:

import { factory, primaryKey, manyOf } from '@mswjs/data'

const db = factory({
  module: {
    id: primaryKey(String),
    lessons: manyOf('lesson')
  },
  lesson: {
    id: primaryKey(String)
  }
})

db.module.create({
  lessons: [
    db.lesson.create({ id: '1' }),
    db.lesson.create({ id: '2' }),
    db.lesson.create({ id: '3' }),
  ]
})

1. module.lessons is always an array. It's either an empty array if you haven't provided any values, or an array with lessons like in the example above. This is the default behavior of manyOf.
2. You cannot update a non-nullable relationship to null/undefined. If you trying doing so, you will get an error:

db.module.update({
  where: { id: { equals: '1' } },
  data: { lessons: undefined }
})

// Error: Failed to update a "MANY_OF" relationship to "lesson
// at "module.lessons" (id: "1"): cannot update a non-nullable relationship to null.

You can also have nullable relationships by wrapping any relationship in nullable. See README for more details.