item-a.yaml

type: object
properties:
  type:
    type: string
    default: a

item-b.yaml

type: object
properties:
  type:
    type: string
    default: b

item.yaml

type: object
properties:
  type:
    type: string
    enum:
      - a
      - b
discriminator:
  propertyName: type
  mapping:
    a: './item-a.yaml'
    b: './item-b.yaml'
required:
  - type

openapi.yaml

openapi: 3.0.0
info:
  version: 1.0.0
  title: Test
tags:
  - name: Resource
    description: |
      <SchemaDefinition schemaRef="#/components/schemas/Resource"/>
  - name: User
    description: |
      <SchemaDefinition schemaRef="#/components/schemas/User"/>
x-tagGroups:
  - name: Output models
    tags:
      - Resource
      - User
components:
  schemas:
    Resource:
      $ref: './resource.yaml'
    User:
      $ref: './user.yaml'

resource.yaml

type: object
required:
  - user
properties:
  items:
    type: array
    items:
      $ref: './item.yaml'

user.yaml

type: object
required:
  - username
properties:
  username:
    type: string