[BUG] Body() descriptions cause generated OpenAPI to use `allOf`instead of schema

Describe the bug When defining a route with a Body parameter, if a description is added to the Body, FastAPI generates an allOf field in the requestBody schema. This breaks OpenAPI Generator, and according to the spec, allOf should be used for object composition.

To Reproduce Create a route with a Body parameter ie:

@route.patch("/{request_id}")
def update_request(request_id: UUID, update:RequestModel = Body(..., description="Blah blah")
     return "stuff"

OpenAPI Result:

        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "title": "Update",
                "allOf": [
                  {
                    "$ref": "#/components/schemas/RequestModel"
                  }
                ],
                "description": ""Blah blah"
              }
            }
          },
          "required": true
        },

Removing the description generates the requestBody without the allOf (and without the description).

Expected behavior

        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/RequestModel"
               "description": "Blah blah"
              }
            }
          }

**Environment:**
 - OS: macOS
 - FastAPI Version 0.42.0
 - Python 3.7.4

[reynoldsdj]

It’s part of the OpenAPI spec. When using $ref, the object containing it cannot contain any additional properties. So the only way to have a $ref and a description is through allOf.

This object cannot be extended with additional properties and any properties added SHALL be ignored.

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#fixed-fields-19

Thank you for the response. I missed that in the OpenAPI specs.

[tiangolo]

It’s part of the OpenAPI spec. When using $ref, the object containing it cannot contain any additional properties. So the only way to have a $ref and a description is through allOf.

This object cannot be extended with additional properties and any properties added SHALL be ignored.

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#fixed-fields-19