Reference API Gateway Models in CloudFormation

API Gateway Models are defined using JSON Schema. You can reference a Model inside another Model using the $ref field. If you are familiar with the OpenAPI Specification fka Swagger, you may have seen something like this:

{
  "$ref": "#/definitions/User"
}

To make model references work with API Gateway, you need to provide an absolute reference that is predefined by AWS: https://apigateway.amazonaws.com/restapis/$ApiId/models/$ModelName

Let’s translate this to a CloudFormation template:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "Lambda and API Gateway",
  "Resources": {
    "RestApi": {
      "Type": "AWS::ApiGateway::RestApi",
      [...]
    },
    "UserModel": {
      "Type": "AWS::ApiGateway::Model",
      "Properties": {
        "ContentType": "application/json",
        "Name": "User",
        "RestApiId": {"Ref": "RestApi"},
        "Schema": {
          "$schema": "http://json-schema.org/draft-04/schema#",
          "type": "object",
          "title": "User",
          "additionalProperties": false,
          "properties": {
            "uid": {
              "type": "string"
            },
            "email": {
              "type": "string"
            },
            "phone": {
              "type": "string"
            }
          },
          "required": ["uid", "email", "phone"]
        }
      }
    },
    "UsersModel": {
      "Type": "AWS::ApiGateway::Model",
      "Properties": {
        "ContentType": "application/json",
        "Name": "Users",
        "RestApiId": {"Ref": "RestApi"},
        "Schema": {
          "$schema": "http://json-schema.org/draft-04/schema#",
          "type": "array",
          "title": "Users",
          "items": {
            "$ref": {"Fn::Join": ["", ["https://apigateway.amazonaws.com/restapis/", {"Ref": "RestApi"}, "/models/", {"Ref": "UserModel"}]]}
          }
        }
      }
    },
    "RestApi": {
      "Type": "AWS::ApiGateway::RestApi",
      "Properties": {
        "Name": "API Gateway ToDo"
      }
    }
  }
}