Swagger complex response model with dynamic key value hash maps

Question

I'm struggling with the syntax of swagger to describe a response type. What I'm trying to model is a hash map with dynamic keys and values. This is needed to allow a localization. The languages may vary, but english should always be provided.

The response would look like this in JSON:

{
  id: "1234",
  name: {
    en: "english text",
    de: "Deutscher Text"
  }
}

My first try was looking like that, but I have no idea how to write the part for the name. AdditionalProperties seems to be a key, but I can't wrap my head around it. Also the requirement for english text is a mystery to me in this syntax and the example also does not seem to work as expected. It generates an empty $folded: in the UI.

delayReason:
  type: object
  properties:
    id:
      type: string
      description: Identifier for a delay reason.
    name:
      type: object
      additionalProperties: 
        type: string
  required: [id, name]
  example:
    id: 123
    name: 
      en: english text
      de: Deutscher Text

But this produces:

There is also no clue in this that the result will have a language code as a key and the text as the value of the hash map.

Answer 1

Your usage of additionalProperties is correct and your model is correct.

additionalProperties

In Swagger/OpenAPI, hashmap keys are assumed to be strings, so the key type is not defined explicitly. additionalProperties define the type of hashmap values. So, this schema

type: object
additionalProperties: 
  type: string

defines a string-to-string map such as:

{
  "en": "English text",
  "de": "Deutscher Text"
}

If you needed a string-to-integer map such as:

{
  "en": 5,
  "de": 3
}

you would define additionalProperties as having value type integer:

type: object
additionalProperties: 
  type: integer

Required key in a hashmap

To define en as a required key in the hashmap:

type: object
properties:
  en:
    type: string
required: [en]
additionalProperties: 
  type: string

Complete example

definitions:
  delayReason:
    type: object
    properties:
      id:
        type: string
        description: Identifier for a delay reason.
      name:
        type: object
        description: A hashmap with language code as a key and the text as the value.
        properties:
          en:
            type: string
            description: English text of a delay reason.
        required: [en]
        additionalProperties: 
          type: string
    required: [id, name]
    example:
      id: '123' # Note the quotes to force the value as a string
      name: 
        en: English text
        de: Deutscher Text

There is also no clue in this that the result will have a language code as a key and the text as the value of the hash map.

Things like that can be documented verbally in the description.

the example also does not seem to work as expected. It generates an empty $folded: in the UI.

Not sure what the problem was with your original spec, but the spec above is valid and looks fine in the Swagger Editor.

Answer 2

It seems you are running into at least three separate bugs and/or limitations:

1. Neither Swagger-Editor nor Swagger-UI provide any indication in the documentation format to show that additionalProperties are allowed in your object schema. So even where you've used additionalProperties correctly, and it's recognized by the Swagger parser, these documentation formats won't show it. You need to add this detail to your schema description, so users understand that they can include additional string properties.
   
   Note: You probably also expect the additional property names to follow a convention, e.g. a two-letter language code. While full JSON Schema lets you specify this with patternProperties, unfortunately that's not supported in Swagger's schema object. So again, you should specify that in your schema description.

2. The Swagger-Editor format sometimes shows this odd "folded:" property. I saw it this morning, now strangely I cannot reproduce it. It might have been hotfixed today. But regardless, it's certainly a bug, and specific to Swagger-Editor. It should not affect your downstream code generation, nor the standard Swagger-UI that presents your API documentation to client developers at runtime. (Though the documentation pane in Swagger-Editor looks similar to Swagger-UI, it is a separate implementation.)

3. There are some subtle but significant limitations to the use of additionalProperties in Swagger. While Helen's example doesn't show any visible errors, in fact the Swagger parser will fail to process this schema correctly; it will ignore either your explicitly declared en property, or will ignore additionalProperties!

This last issue comes down to a design flaw in Swagger-Model, one of the core components used throughout the Swagger Java stack (including Swagger-Codegen). Schemas defined in certain contexts can work with a combination of properties and additionalProperties. But schemas defined in other contexts cannot.

We've documented this in detail here.

The good news: with a small tweak, we can make Helen's example work correctly. We just need to extract the nested object schema into its own top-level definition. I'll call it LocalizedName:

definitions:
  delayReason:
    type: object
    properties:
      id:
        type: string
        description: Identifier for a delay reason.
      name:
        $ref: "#/definitions/LocalizedName"
    required: [id, name]
    example:
      id: '123' # Note the quotes to force the value as a string
      name: 
        en: English text
        de: Deutscher Text

  LocalizedName:
    type: object
    description: A hashmap with language code as a key and the text as the value.
    properties:
      en:
        type: string
        description: English text of a delay reason.
    required: [en]
    additionalProperties: 
      type: string