Swagger: map of <string, Object>

Question

I need to document with Swagger an API that uses, both as input and output, maps of objects, indexed by string keys.

Example:

{
    "a_property": {
        "foo": {
            "property_1": "a string 1",
            "property_2": "a string 2"
        },
        "bar": {
            "property_1": "a string 3",
            "property_2": "a string 4"
        }
    }
}

"foo" and "bar" can be any string keys, but they should be unique among the set of keys.

I know that, with Swagger, I can define an array of objects, but this gives a different API since we then would have something as:

{
    "a_property": [
        {
            "key": "foo"
            "property_1": "a string 1",
            "property_2": "a string 2"
        },
        {
            "key": "bar"
            "property_1": "a string 3",
            "property_2": "a string 4"
        }
    ]
}

I have read the 'Open API Specification' - 'Add support for Map data types #38' page. As far as I understand, it recommends to use additionalProperties, but it doesn't seem to answer my need (or it doesn't work with Swagger UI 2.1.4 that I use). Did I miss something?

So far I have found the following work-around (in Swagger JSON):

a_property: {
    description: "This is a map that can contain several objects indexed by different keys.",
    type: object,
    properties: {
        key: {
            description: "map item",
            type: "object",
            properties: {
                property_1: {
                    description: "first property",
                    type: string
                },
                property_2: {
                    description: "second property",
                    type: string
                }
            }
        }
    }
}

This almost does the job, but the reader has to understand that "key" can be any string, and can be repeated several times.

Is there a better way to achieve what I need?

Personally it took me some time to understand why additionalProperties is the right answer: stackoverflow.com/a/41240118/110488 — Chen Levy

Arnaud Lauret Arnaud Lauret · Accepted Answer · 2016-05-16T16:03:18

Using additionalPropertiesis the proper way to describe hashmap with OpenAPI (fka. Swagger) Specification but Swagger UI do not render them for now.

The issue is tracked here https://github.com/swagger-api/swagger-ui/issues/1248

Meanwhile you can use this trick: define a non required property (defaultin the example below) of the same type of the map's objects and give some hint within the description:

swagger: "2.0"
info:
  version: 1.0.0
  title: Hashmap
  
paths: {}

definitions:
  MapItem:
    properties:
      firstname:
        type: string
      lastname:
        type: string
  Map:
    description: a (key, MapItem) map. `default`is an example key
    properties:
      default:
        $ref: '#/definitions/MapItem'
    additionalProperties:
      $ref: '#/definitions/MapItem'

This description does not modify API contract and improves documentation.

Swagger: map of <string, Object>

2 Answers