[BUG] Discriminator mapping not correct when using oneOf with discriminator

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
  yes
• Have you validated the input using an OpenAPI validator (example)?
  yes
• What's the version of OpenAPI Generator used?
  4.3.1
• Have you search for related issues/PRs?
  yes

Description

The generated code leads to the following result:

@JsonInclude(JsonInclude.Include.NON_NULL)
@javax.annotation.processing.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", date = "2020-06-25T13:58:21.826225400+02:00[Europe/Berlin]")
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "parentId", visible = true)
@JsonSubTypes({
@JsonSubTypes.Type(value = TestHolder.class, name = "whatsup"),
@JsonSubTypes.Type(value = WhatHolder.class, name = "tst"),
})

public interface ArrayItem {
public String getParentId();
}

I would expect the following result

@JsonInclude(JsonInclude.Include.NON_NULL)
@javax.annotation.processing.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", date = "2020-06-25T13:58:21.826225400+02:00[Europe/Berlin]")
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "parentId", visible = true)
@JsonSubTypes({
@JsonSubTypes.Type(value = TestHolder.class, name = "whatsup"),
@JsonSubTypes.Type(value = WhatHolder.class, name = "tst"),
})

public interface ArrayItem {
public ParentIdEnum getParentId();
}

It should use the correct type from the child!.

See content of YAML

OpenAPI declaration file content or url

openapi: 3.0.0
info:
version: "0.0.0.1"
title: ""
x-apiType: ""
description: ""
paths:
/test:
post:
tags:
- "test"
description: "tes"
operationId: "test"
requestBody:
description: "Payload for premium calculation."
required: true
content:
application/json:
schema:
$ref: "#/components/schemas/NewRequest"
responses:
"200":
description: "Successful operation"
components:
schemas:
Parent:
required:
- parentId
type: object
properties:
parentId:
type: string
enum:
- whatsup
- tst
discriminator:
propertyName: parentId
mapping:
whatsup: "#/components/schemas/What"
tst: "#/components/schemas/Test"
What:
type: object
allOf:
- $ref: "#/components/schemas/Parent"
properties:
whatever:
type: string
Test:
type: object
allOf:
- $ref: "#/components/schemas/Parent"
properties:
test:
type: string
NewRequest:
type: object
properties:
parents:
type: array
minItems: 1
maxItems: 100
items:
$ref: "#/components/schemas/arrayItem"
arrayItem:
type: object
oneOf:
- $ref: "#/components/schemas/TestHolder"
- $ref: "#/components/schemas/WhatHolder"
discriminator:
propertyName: parentId
mapping:
whatsup: "#/components/schemas/TestHolder"
tst: "#/components/schemas/WhatHolder"
TestHolder:
type: object
allOf:
- $ref: "#/components/schemas/What"
properties:
testHolder:
type: string
WhatHolder:
type: object
allOf:
- $ref: "#/components/schemas/Test"
properties:
whatHolder:
type: string

Start by reproducing the issue with the supplied OpenAPI YAML and the Java client generator, then inspect the generated ArrayItem interface and its discriminator handling. Compare the generated parentId type with the expected ParentIdEnum type; done means the discriminator-based oneOf models generate the correct child-derived type.