Creating an extendible model using Swagger/ OpenAPI

Question

In my API i would like to have a simple model for my collection and a more elaborate model for my individual resource. For example:

a GET request on /libraries should return

BaseLibrary:
    type: object
    properties:
        library_id:
          type: string
          description: The id of the library
        display_name:
          type: string
          description: Name of the library
        href:
          type: string
          description: The URI linking to this library.

whilst a request to a specific library should return all of the above including an extra parameter books:

So a GET request to libraries/{library_id} should return:

ExtendedLibrary:
    type: object
    properties:
        library_id:
          type: string
          description: The id of the library
        display_name:
          type: string
          description: Name of the library
        href:
          type: string
          description: The URI linking to this library.
        books:
          type: array
          description: The books in this library
          items:
            $ref: "#/definitions/books"

I would very much like to not have to define a "BaseLibrary" twice and would want to simple model an additional "ExtendedLibrary" which contains all the responses of a base library and the additional books property.

I tried a lot of different things, with the closest to succes being the following definitions:

definitions:
  BaseLibrary:
    type: object
    properties:
        library_id:
          type: string
          description: The id of the library.
        display_name:
          type: string
          description: Name of the library
        href:
          type: string
          description: The URI linking to this library.

  ExtendedLibrary:
    type: object
    properties:
      $ref: "#/definitions/BaseLibrary/properties"
      books:
        type: array
        description: The available books for this library.
        items:
          $ref: "#/definitions/Book"

However this gives me a "Extra JSON reference properties will be ignored: books" warning and the output seems to ignore this extra property. Is there a clean way to handle my problem? Or am I just going to have to copy paste my whole BaseLibrary model into my ExtendedLibrary model?

Answer 1

As mentioned in the comments section, this may be a duplicate of another question, but it's worth repeating the answer in the context of this particular example. The solution is to use the allOf property in the definition of ExtendedLibrary:

definitions:
  Book:
    type: object
    properties:
      title:
        type: string
      author:
        type: string

  BaseLibrary:
    type: object
    properties:
      library_id:
        type: string
        description: The id of the library
      display_name:
        type: string
        description: Name of the library
      href:
        type: string
        description: The URI linking to this library.

  ExtendedLibrary:
    type: object
    allOf:
      - $ref: '#/definitions/BaseLibrary'
      - properties:
          books:
            type: array
            description: The books in this library
            items:
              $ref: "#/definitions/Book"

In my experience, Swagger UI visualizes this correctly. When I define an operation response to be ExtendedLibrary Swagger UI shows this example:

{
  "library_id": "string",
  "display_name": "string",
  "href": "string",
  "books": [
    {
      "title": "string",
      "author": "string"
    }
  ]
}

Also, Swagger Codegen does the right thing. At least when generating a Java client, it creates an ExtendedLibrary class that correctly extends BaseLibrary.