Fix siblings of $ref using allOf in openapi normalizer

[wing328]

update normalizer to handle siblings of schema with $ref by converting it to allOf, e.g. normalize

        refProperty:
          description: Ref-Property-Description
          $ref: '#/components/schemas/RefObject'

to

        refProperty:
          description: Ref-Property-Description
          allOf:
            - $ref: '#/components/schemas/RefObject'

Close #20304

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[jpfinne]

@wing328 You've tested with simple types (type: string). I think you introduce breaking changes for complex types.

Example:

components:
  schemas:
    Base:
      description: Base Description
      type: object
      properties:
        id:
          type: integer
    BaseWithDescription:
      description: Base With Description
      $ref: '#/components/schemas/Base'

    Parent:
      description: Parent description
      type: object
      x-parent: true
      properties:
        id:
          type: integer
    Child:
      description: Child With Description
      $ref: '#/components/schemas/Parent'

I've tested with the spring generator.
In 7.4.0, 2 classes were generated: Base.java and Parent.java
After this PR, 4 classes are generated: Base.java, Parent.java, BaseWithDescription.java and Child.java (class Child extends Parent)

ModelUtils.isComplexComposedSchema(Schema) could be used to discard the complex types (eventually configurable).

I have a question about the use of allOf.
For simple type, you could inline the sibling type. Isn't it simpler for the generators?

RefObject:
   type: string
refProperty:
   description: Ref-Property-Description
   $ref: '#/components/schemas/RefObject'

becomes:

RefObject:
   type: string
refProperty:
   description: Ref-Property-Description
   type: string

[wing328]

In 7.4.0, 2 classes were generated: Base.java and Parent.java

7.4.0 was released a while ago. Are you able to reproduce the issue with v7.17.0? just want to confirm the issue is with this change not earlier stable version v7.17.0

and thanks for testing it. i'll take another look.

[jpfinne]

Sorry, It is indeed a regression from 7.16.0.

[wing328]

about use of allOf, please refer to https://stackoverflow.com/a/51402417/677735 for more info.

We support the openapi 3.0.x way of overriding fields and that's why openapi normalizer normalizes schemas (e.g. 3.1) to conform to this way so that default codegen can handle these schemas with ease.

[wing328]

Sorry, It is indeed a regression from 7.16.0.

No need to sorry and thanks for testing it.

[jpfinne]

@wing328 the stackoverflow article descibes a nice way to patch an openapi 3,0 contract so that it has features only available in 3.1. The swagger parser does not keep attributes like description when there is a $ref in 3.0.

In this PR you do the opposite. You convert an openapi 3.1 contract to a 3.0 contract while keeping the functionality.

It would be nice if the code generation supports the pattern

    refProperty:
      description: Ref-Property-Description
      allOf:
        - $ref: '#/components/schemas/RefObject'

But unfortunately it does not !

I've tested this PR with the spring generator:

openapi: '3.1.0'
components:
  schemas:
    RefObject:
      type: string
    refProperty:
      description: Ref-Property-Description
      $ref: '#/components/schemas/RefObject'
    Base:
      type: object
      properties:
        refProperty:
          $ref: '#/components/schemas/refProperty'

It generates an Object instead of a String in pre-PR.

public class Base {

  private @Nullable Object refProperty;

In the normalizer, there is already a parsed OpenAPI. So the trick is not really needed.
Inlining is an option for simple types.