It's impossible to send request with selected value by default

[AlexeyKorkoza]

I'm submitting a...

[ ] Regression 
[x] Bug report
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

I was redirected to here by the author of the swagger-ui-express (scottie1984/swagger-ui-express#219 (comment))
So, below is my description.

Current behavior

I have an endpoint that allows me to upload stats for different types of sports matches and select the type. Right now, I can upload the statistics file only for hockey matches that's why only one item is added to the select. So, I attach the statistics file and press on the execute button as the select has only one item. However, the request isn't sent to my backend via the swagger. I can see that the border of the select is red colour and something went wrong but what?
I attached the screenshot below the view of my endpoint.

I would say before creating this issue I was looking through various issues on both stackoverflow and here. Unfortunately, I couldn't find a solution. I read through the documentation but the result is the same.

Expected behavior

I expect I will be able to send a request to my backend to upload stats without any issues in spite of that the select has only one item now.

Minimal reproduction of the problem with instructions

I decided to create the repository where I reproduced my issue.
Link: https://github.com/AlexeyKorkoza/nestjs-swagger-reproduce-issue

Environment

Nest version: 7.5.1

 
For Tooling issues:
- Node version: 12.16.3
- Platform:  Windows

Others:
Operating system version: Windows 10 Pro
OS build: 19041.450
IDE: Webstorm 2020.1.4
Package manager: yarn, 1.22.4

[hkosova]

Please post your API definition file (YAML or JSON). You can export it from Swagger UI as explained here: https://stackoverflow.com/q/48525546.

[AlexeyKorkoza]

@hkosova
The definition file:

{
  "openapi": "3.0.0",
  "info": {
    "title": "",
    "description": "",
    "version": "1.0",
    "contact": {}
  },
  "tags": [],
  "servers": [],
  "components": {
    "schemas": {
      "SportDto": {
        "type": "object",
        "properties": {
          "typeSport": {
            "type": "string",
            "enum": [
              "HOCKEY"
            ]
          },
          "stats": {
            "type": "string",
            "format": "binary"
          }
        },
        "required": [
          "typeSport",
          "stats"
        ]
      },
      "ResponseDto": {
        "type": "object",
        "properties": {}
      }
    }
  },
  "paths": {
    "/sports": {
      "post": {
        "operationId": "AppController_postHello",
        "parameters": [],
        "requestBody": {
          "required": true,
          "content": {
            "multipart/form-data": {
              "schema": {
                "$ref": "#/components/schemas/SportDto"
              }
            }
          }
        },
        "responses": {
          "201": {
            "description": "",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/ResponseDto"
                }
              }
            }
          }
        }
      }
    }
  }
}

If somebody has a NestJS application and wants to export the definition he should add the api-json to the end of the url.
The issue link where is mentioned about it:
nestjs/swagger#158 (comment)

[hkosova]

Confirmed a bug: if an OAS3 form field is required and has a single-value enum, "try it out" doesn't work.

The workaround is to add an example value for the typeSport property:

          "typeSport": {
            "type": "string",
            "enum": [
              "HOCKEY"
            ],
            "example": "HOCKEY"    <-----------
          },