How do I save/create a model with a relation?

[8funtik8]

I have 2 models - User and Promo code.
As stated in the documentation (https://docs.adonisjs.com/guides/models/relationships#hasmany) I specified in the Promo code model the connection with the User:

Models/PromoCode.ts

export default class PromoCode extends BaseModel {
  @column({ isPrimary: true })
  public id: number

  @column()
  public code: string

  @column()
  public is_active: boolean

   ////////////////// Here
  @hasMany(() => User, {
    foreignKey: 'id',
  })
  public users: HasMany<typeof User>

  @column.dateTime({ autoCreate: true })
  public created_at: DateTime

  @column.dateTime({ autoCreate: true, autoUpdate: true })
  public updated_at: DateTime
}

In the User model migration file:

public async up() {
    this.schema.createTable(this.tableName, (table) => {
      table.increments('id').primary()
      table.string('username', 50).unique().notNullable()
      table.string('email', 255).unique().notNullable()
      table.string('password', 255).notNullable()
      table.string('remember_me_token').nullable()

      table.timestamp('created_at', { useTz: true }).notNullable()
      table.timestamp('updated_at', { useTz: true }).notNullable()

      table.integer('payments').unsigned()
      table.foreign('payments').references('id').inTable('payments').onDelete('CASCADE')

      ////////////////// Here
      table
      .integer('promo_code_id')
      .unsigned()
      .references('promo_codes.id')
      .defaultTo(null)
      .onDelete('CASCADE')
    })
  }

I didn't specify anything else to link the two models.
Models/User.ts

export default class User extends BaseModel {
  
  @column({ isPrimary: true })
  public id: number

  @column()
  public username: string

  @column()
  public email: string

  @column({ serializeAs: null })
  public password: string

  @beforeSave()
  public static async hashPassword(user: User) {
    if (user.$dirty.password) {
      user.password = await Hash.make(user.password)
    }
  }

  @column()
  public remember_me_token?: string

  @column()
  public remaining_requests: number

  @column()
  public remaining_exports: number

  @hasMany(() => Payment)
  public payments: HasMany<typeof Payment>

  @column.dateTime({ autoCreate: true })
  public created_at: DateTime

  @column.dateTime({ autoCreate: true, autoUpdate: true })
  public updated_at: DateTime
}

In the PromoCode model migration file:

public async up () {
    this.schema.createTable(this.tableName, (table) => {
      table.increments('id').primary()
      table.string('code', 50).notNullable()
      table.boolean('is_active').notNullable().defaultTo(true)

      table.timestamp('created_at', { useTz: true }).notNullable()
      table.timestamp('updated_at', { useTz: true }).notNullable()
    })
  }

How do I create or save a model with a relation: one promo code -> many users?
I start doing this, but then I can't figure out how to tie everything together?

export default class CreateUser extends BaseCommand {
  public static commandName = 'create:user'
  public static description = 'Create a new user'
  public static settings = {
    loadApp: true
  }

  public async run () {
    const email = await this.prompt.ask('Enter email')
    const username = await this.prompt.ask('Enter username')
    const password = await this.prompt.secure('Choose account password')
    const code = await this.prompt.ask('Enter promo code')

    const { default: User } = await import('App/Models/User')
    const { default: PromoCode } = await import('App/Models/PromoCode')
    const promo_code = await PromoCode.findBy('code', code)
    const user = await User.create({email: email, username: username, password: password})
  }

How should I save/create/update a model instance with a relation?

[ThomasWeber]

At a quick glance one thing I see that seems to be missing is a BelongsTo relation defined in the User model.
https://docs.adonisjs.com/guides/models/relationships#belongsto

[8funtik8]

Thank you so much. Yes, I forgot to include this in User model.

@column()
public promo_code_id: number | null

@belongsTo(() => PromoCode)
public promo_code: BelongsTo<typeof PromoCode>

This is the last thing I noticed in the documentation

@column()
public promo_code_id: number | null

Now I have an understanding of my mistake.