Hey Arthur! I just gave it a go and everything seems to be working a-okay with using decorators in a mixin! Only caveat is, it looks like you have to give the class a name and return it via that name on a separate line for TypeScript to be happy. Below is what I tried, note the imports are for AdonisJS 6, but the functionality would be the same between 5 & 6.

import { BaseModel, belongsTo, column } from "@adonisjs/lucid/orm"
import { NormalizeConstructor } from '@adonisjs/core/types/helpers'
import Organization from "#models/organization"
import type { BelongsTo } from "@adonisjs/lucid/types/relations"

export const WithOrganization = <T extends NormalizeConstructor<typeof BaseModel>>(superclass: T) => {
  class parentclass extends superclass {
    @column()
    declare organizationId: number

    @belongsTo(() => Organization)
    declare organization: BelongsTo<typeof Organization>
  }

  return parentclass
}

  

    

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app
models
mixins
organization.ts

import { DateTime } from 'luxon'
import { BaseModel, column, hasMany } from '@adonisjs/lucid/orm'
import type { HasMany } from '@adonisjs/lucid/types/relations'
import Course from '#models/course'
import { compose } from "@adonisjs/core/helpers"
import { WithOrganization } from "#models/mixins/organization"

export default class Difficulty extends compose(BaseModel, WithOrganization) {
  @column({ isPrimary: true })
  declare id: number

  @column()
  declare name: string
}

  

    

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app
models
difficulty.ts

As a heads up, you can define a parent/child relationship directly on the model without using compose. It'll look like the below.

export default class Taxonomy extends BaseModel {
  // other columns

  @column()
  declare parentId: number | null // parent taxonomy's id (null for top-level)

  @belongsTo(() => Taxonomy, {
    foreignKey: 'parentId', // use parentId to define parent relationship
  })
  declare parent: BelongsTo<typeof Taxonomy>
}

  

    

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Hope this helps!
—-
Edit: Updated to account for the TypeScript requirement of the mixin class needing a name and for the return to be on a separate line. I missed the small red squiggly on the decorator when I first attempted this.

Hi Danilesky17! Hmm, yeah it seems to have slipped from the documentation. Luckily though, it is still within AdonisJS, it's just now under the core helpers. Below is the example from this lesson updated for AdonisJS 6!

import { BaseModel } from '@adonisjs/lucid/orm'
import { NormalizeConstructor } from '@adonisjs/core/types/helpers'

export const WithExtras = <T extends NormalizeConstructor<typeof BaseModel>>(superclass: T) => {
  return class extends superclass {
    serializeExtras: boolean = true

    get meta() {
      return this.$extras
    }
  }
}

  

    

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import { compose } from '@adonisjs/core/helpers'
import { WithExtras } from './mixins/serialize.js'
import { BaseModel, column, hasMany } from '@adonisjs/lucid/orm'

export default class Topic extends compose(BaseModel, WithExtras) {
  @column({ isPrimary: true })
  declare id: number

  @column()
  declare name: string
}

  

    

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Hope this helps!