Have arrays within arrays in a Swagger responses

I am trying to define an API in Swagger where the result of a GET is an array of items for model MainModel each with an associated array of items from another model called AnotherModel. Something like this (which does not seem to be allowed):

responses:
    200:
      description: search results matching criteria
      schema:
        type: array
        items:
          $ref: '#/definitions/MainModel'
          type: array
          items: $ref: '#/definitions/AnotherModel'

Solution

First of all, OpenAPI Specification supports associative arrays / dictionaries with string keys only, such as:

{
  "foo": 5,
  "bar": 2
}

but not C#'s Dictionary<int, string> where keys are int.

An associative array is defined by using an object schema with the additionalProperties keyword specifying the type of array values. The key type is not mentioned because keys are always strings.

type: object
additionalProperties:
  type: <type of array values>
  # OR
  # $ref: '#/definitions/ValueModel'

Similarly, an array of associative arrays can be defined as follows:

type: array
items:
  type: object
  additionalProperties:
    type: <type of values in associative array>
    # OR
    # $ref: '#/definitions/ValueModel'

In your example, the values of associative array are of type AnotherModel. So an inline definition would look like:

      responses:
        200:
          description: search results matching criteria
          schema:
            type: array
            items:
              type: object
              additionalProperties:
                $ref: '#/definitions/AnotherModel'

definitions:
  AnotherModel:
    type: <whatever> # TODO

Or if we introduce an intermediate MainModel that represents the associative array (as in your original example):

      responses:
        200:
          description: search results matching criteria
          schema:
            type: array
            items:
              $ref: '#/definitions/MainModel'

definitions:
  MainModel:  # Associative array of AnotherModel
    type: object
    additionalProperties:
      $ref: '#/definitions/AnotherModel'
  AnotherModel:
    type: <whatever> # TODO

A couple of notes:

Each model referenced via $ref must be defined in the definitions section. So if you use $ref: '#/definitions/MainModel', your spec must include
```
  definitions:
    MainModel:
```
$ref works by replacing itself and all its sibling elements with the definition it is pointing at. That's why your original example is not valid:
```
  items:
    $ref: '#/definitions/MainModel'
    type: array
    items: $ref: '#/definitions/AnotherModel'
```
Nested models can be either defined completely inline or referenced via $ref, but not both.