[BUG] Not possible to ever read object that sent as query parameter

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [] [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Using OpenApi 3.0 and defining a query parameter that has object as value:

paths:
  /archived-documents:
    get:
      tags:
        - "Archived documents"

      operationId: "getArchivedDocuments"

      parameters:
        - $ref: "#/components/parameters/documentId"
        - $ref: "#/components/parameters/documentMetadata"

...
    
    documentMetadata:
      name: documentMetadata
      in: query
      schema:
        $ref: "#/components/schemas/documentMetadata"

....

schemas:
    documentMetadata:
      description: The metadata associated with the document as key/value pairs
      type: object
      additionalProperties:
        type: object
      example:
        metadataKey1: metadataValue1
        metadataKey2: metadataValue2
        metadataKey3: metadataValue3

The Maven plugin will generate server code that I use in my Spring Boot project.

The specific field is generated as:

  @JsonProperty("documentMetadata")
  @Valid
  private Map<String, Object> documentMetadata = null;

Every other fields works OK except the documentMetadata field. There seems to be no way of sending this as query parameter to the server and get it properly made available in the code. My expectation is that the key/value-pairs are populated in this map.

I have tried various ways to send data to the server:

http://myapp/myservice?param1=A&param2=B&documentMetadata[lookupkey]=123

This one fails with illegal character. If I URL encode it, the field on serverside has null value.

http://myapp/myservice?param1=A&param2=B&documentMetadata={"partnerId":"123"} (and also URL encoded tried)

This one gives me error that it cannot cast String to Map: "Cannot convert value of type 'java.lang.String' to required type 'java.util.Map': no matching editors or conversion strategy found]"

openapi-generator version

			<groupId>org.openapitools</groupId>
			<artifactId>openapi-generator-maven-plugin</artifactId>
			<version>4.2.3</version>

OpenAPI declaration file content or url

See above.

Generation Details

No special usage.

Steps to reproduce

Create openapi 3 Swagger file as shown above. Generate code. Observe that there are no way to actually send these values to the server.

Related issues/PRs

Suggest a fix

Start with the OpenAPI 3 query-parameter generation used by the Maven plugin for Spring Boot server code, using the provided documentMetadata object schema and request examples as the reproduction. Trace how the generated Map<String, Object> parameter is bound from query input, then verify that key/value pairs are available on the server without String-to-Map conversion errors.