[BUG] External schemas referenced from the content of a parameter are not found

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

If a parameter has an external reference inside of the content field, the generator will not be able to get the schema from the reference. It works if the reference is towards a schema in the components of the file where the parameter is defined.

openapi-generator version

4.0.0, commit 67423029225e003a1bdeb29909a960ae7e16ed22

OpenAPI declaration file content or url

test.txt

openapi: '3.0.0'
info:
  version: 1.0.0
  title: Test
paths: 
  /:    
    get:
      parameters:
        - name: test
          in: query
          content:
            'application/json':
              schema:
                $ref: './external_components.yaml#/components/schemas/Ref'
      responses:
        default:
          description: Unexpected error

external_components.yaml

openapi: 3.0.0
info:
  version: '1.0.0'
  title: 'External definition for schemas'
paths: {}
components:
  schemas:
    Ref:
      type: string

Command line used for generation

java -jar openapi-generator-cli.jar generate -i test.yaml -g csharp-nancyfx -o out/ >> log.txt

Steps to reproduce

• Create a file test.yaml with the first specification
• Create a file external_components.yaml with the second specification in the same folder as test.yaml
• Put openapi-generator-cli.jar in the same folder as the two yaml files
• Run the command
• Open log.txt

The log file will contain the following warning:

[main] WARN  o.o.codegen.DefaultCodegen - Error obtaining the datatype from ref:./external_components.yaml#/components/schemas/Ref. Default to 'object'

The expected result would be to not have that working and have the parameter use the schema named Ref rather than object.

Related issues/PRs

I haven't found any

Suggest a fix

I tried to look all around the source code but I couldn't find the reason for it, as the content is processed correctly for both requestBody and responses.

Start at DefaultCodegen, where the warning reports failure to resolve the reference, and compare parameter content handling with the working requestBody and responses paths. Reproduce with test.yaml and external_components.yaml using the csharp-nancyfx generation command; done means the external Ref schema is resolved and the parameter uses it instead of object.