fix: keep oneOf when WRAPPER_OBJECT wrapping is applied

[vpelikh]

Summary

A schema that already carries a oneOf now keeps it when Jackson's WRAPPER_OBJECT type info is applied. Previously the composition was silently dropped and the schema became a plain object with a single property.

Problem

ModelResolver.resolveWrapping(...) builds a new wrapper schema and nests the model inside it:

Schema wrapperSchema = openapi31 ? new JsonSchema().typesItem("object") : new ObjectSchema();
wrapperSchema.name(model.getName());
wrapperSchema.addProperties(name, model);
return wrapperSchema;

The wrapper copies only the name and the single property. If model had a oneOf (from @Schema(oneOf = ...), @Schema(subTypes = ...) or @JsonSubTypes), it is lost: the returned schema is an ObjectSchema, not a ComposedSchema, and the oneOf is gone.

Reproduction (@JsonTypeInfo(include = WRAPPER_OBJECT) + @Schema(oneOf)):

	result
before	ObjectSchema, properties [Pet], no oneOf
after	ComposedSchema, oneOf = 2 refs, wrapper property present

Change

When the model already has a non-empty oneOf, add the wrapper property directly to the model and return it, instead of nesting it in a wrapper schema:

if (model.getOneOf() != null && !model.getOneOf().isEmpty()) {
    model.addProperties(name, newObjectSchema());
    return model;
}

The empty-object-schema expression was already used once in this method, so instead of repeating it a second time it is extracted to a small helper:

/** An empty object schema for the current OpenAPI version. */
private Schema newObjectSchema() {
    return openapi31 ? new JsonSchema().typesItem("object") : new ObjectSchema();
}

Behavior is unchanged for models without oneOf.

Tests

Adds WrapperObjectOneOfTest. It fails before the change (ObjectSchema with no oneOf) and passes after it.

Notes

This is the small stack split out of #5320. It is a standalone bug fix, reproducible on unmodified 3.0.0, and does not depend on @JsonSubTypes work.

[ewaostrowska]

Thank you for the PR @vpelikh!

I tested this and pushed a small change on top: I removed the empty placeholder property, because it added noise to the schema. I also added tests (no extra property next to oneOf, resolving the same class twice, OpenAPI 3.1, and a model without oneOf is still wrapped as before).

Before the PR, Pet was nested one level down under the wrapper key:

Pet:
  type: object
  properties:
    Pet:
      type: object
      properties:
        petType: {type: string}
      oneOf: [Dog, Cat]

After the PR, as submitted:

Pet:
  type: object
  properties:
    petType: {type: string}
    Pet: {type: object}   # empty placeholder
  oneOf: [Dog, Cat]

The oneOf is back at the top level, which is what tools expect for a polymorphic type, so this is an improvement. But Pet: {type: object} describes nothing and does not match the real JSON, so I removed it. The model is now returned unchanged when it has a oneOf. This also avoids changing the model in place.

The schema is still not fully correct. With WRAPPER_OBJECT, the JSON is:

{"Dog": {"petType": "dog", "bark": "woof"}}

To match it, each oneOf branch should be wrapped, not the base type:

Pet:
  oneOf:
    - type: object
      required: [Dog]
      properties:
        Dog: {$ref: '#/components/schemas/Dog'}
    - type: object
      required: [Cat]
      properties:
        Cat: {$ref: '#/components/schemas/Cat'}

That needs a more complex handler (subtype names, $ref targets, 3.0 and 3.1) and more tests, so I think it is out of scope here. I added a TODO in the code.

[vpelikh]

Hey @ewaostrowska. Thanks for reviewing this and other PRs. Are you okay with this PR as is, or would you like me to make some changes?

[ewaostrowska]

@vpelikh I think the current state should be fine for now, but let's wait on review from @djankows or @daniel-kmiecik

[Copilot]

🟡 Changes recommended

The implementation preserves oneOf by removing required WRAPPER_OBJECT semantics, producing a schema that does not match serialized values.

1 open finding

• Preserve WRAPPER_OBJECT semantics in composed subtype schemas · New

What changed in this PR

Attempts to preserve oneOf schemas during Jackson WRAPPER_OBJECT resolution.

Changes:

• Bypasses wrapper creation when oneOf is present.
• Adds regression tests for OpenAPI 3.0 and 3.1.

File	Description
ModelResolver.java	Alters wrapping behavior for composed schemas.
WrapperObjectOneOfTest.java	Tests oneOf preservation and repeated resolution.

🧠 Review effort: Balanced

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.