[BUG] [Parser] ref's are handled improperly inside additionalProperties if the schema is an array and using multiple files

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When having a $ref inside an array inside an additionalProperties section the parser doesn't seem to pick up the definition properly - especially when using multiple files

For example let's say you want to describe a schema like the following:

{
  "validations": {
    "first_name": [{
      "presence":true
    }]
  }
}

Where the validations object can contain a wildcard of properties - each of them an array of another specific schema. See the example below for a minimum failing case

openapi-generator version

Tested with 0.4.0-beta2 and the latest cli from docker

OpenAPI declaration file content or url

Check the following files:

# ==> api.yaml <==
openapi: 3.0.2
servers:
  - url: 'https://example.com'
info:
  title: Example API
  version: 1.0
paths:
  /test:
    $ref: './endpoints/endpoint.yaml#/collection'

# ==> definitions/definition.yaml <==
TestResponse:
  additionalProperties:
    type: array
    items:
      $ref: '#/TestDescription'

TestDescription:
  type: object
  properties:
    error:
      type: string

# ==> endpoints/endpoint.yaml <==
collection:
  get:
    operationId: 'test'
    responses:
      '200':
        content:
          application/json:
            schema:
              $ref: '../definitions/definition.yaml#/TestResponse'

Command line used for generation

docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli generate \
    -i /local/api.yaml \
    -g openapi \
    -o /local/gen

Steps to reproduce

Run the generator against the example provided above.

The generated file will return the following error:

[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: #/TestDescription
[main] WARN  o.o.codegen.utils.ModelUtils - Failed to get the schema name: #/TestDescription

although the TestDescription definition is right there in the file.

Not using an array, e.g.

TestResponse:
  additionalProperties:
    $ref: '#/TestDescription'

works as expected.

Suggest a fix

The following workaround seems to be working (see the two inline comments on what's changed):

# ==> api.yaml <==
openapi: 3.0.2
servers:
  - url: 'https://example.com'
info:
  title: Example API
  version: 1.0
paths:
  /test:
    $ref: './endpoints/endpoint.yaml#/collection'
# duplicating the schemas explicitly here
components:
  schemas:
    TestDescription:
      $ref: './definitions/definition.yaml#/components/schemas/TestDescription'

# ==> definitions/definition.yaml <==
TestResponse:
  additionalProperties:
    type: array
    items:
      $ref: '#/components/schemas/TestDescription'

# duplicating the main structure of components/schemas here as well
components:
  schemas:
    TestDescription:
      type: object
      properties:
        error:
          type: string

# ==> endpoints/endpoint.yaml <==
collection:
  get:
    operationId: 'test'
    responses:
      '200':
        content:
          application/json:
            schema:
              $ref: '../definitions/definition.yaml#/TestResponse'

i.e. defining the model on the main file, and also faking it's location in the sub-files seems to be okay. It is also picked up properly by multiple languages we tried including java, ruby, php and openapi

Start with the multi-file fixture described in api.yaml, endpoints/endpoint.yaml, and definitions/definition.yaml, then run the supplied Docker generation command with the openapi generator. Trace how the parser resolves the $ref under additionalProperties.items and review the ModelUtils warning. Done means the example generates without the Failed to get the schema name warning and resolves TestDescription correctly.