Form-data array properties: support encoding

[lamuertepeluda]

Is there an existing issue that is already proposing this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe it

Hello, I need to generate a form-data input where there are some properties which are arrays.

I found out that the ApiBody decorator does not allow to specify the encoding property defined by OAS 3.0 (search for "encoding:" since this page does not have anchors).

Sadly the Swagger UI client, when the encoding attribute is missing, encodes the array properties as csv strings.

i.e., instead of

formData.add('arrayProp', elem0)
formData.add('arrayProp', elem1)
...
formData.add('arrayProp', elemN)

it does

formData.add('arrayProp', 'elem0,elem1,...,elemN')

Some people added some wonderful nasty workaround like this for NestJS. But it would be great if this was supported by the library.

Describe the solution you'd like

Support the encoding attribute in ApiBody as optional parameter

@ApiBody({
      encoding: {
          arrayProp1: { explode: true, style: 'form' },
          arrayProp2: { explode: true, style: 'form' }
          ...
      },
      schema: {
        ...
         properties: {
           arrayProp1: { type: 'array', items: {type: 'string'} },
           arrayProp2: { type: 'array', items: {type: 'object', properties: {...}} }
         }
      },

The encoding parameter should be optional.

Teachability, documentation, adoption, migration strategy

Should not be a breaking change, but just a new feature

What is the motivation / use case for changing the behavior?

Make the Swagger UI behave correctly when using it to test form data endpoints

Allow generated OpenAPI client to handle this parameter.

[kamilmysliwiec]

Would you like to create a PR for this issue?

[lamuertepeluda]

@kamilmysliwiec I can try

[lamuertepeluda]

@kamilmysliwiec PR here: fba860f

(with some help from Cursor) I also added the tests for api-body-decorator, which were missing.
I tried in a local project and now the Swagger UI works as expected.