[typescript-fetch] support explode query parameters

[bobvanderlinden]

Related to #3781

Currently multiple query parameters that are serialized as CSV. param: [1,2,3] becomes param=1,2,3.

The OpenAPI specification https://swagger.io/docs/specification/serialization/#query says the default serialization for query parameters with multiple values is style: form with explode: true. That means that means that serialization for param: [1,2,3] should be param=1&param=2&param=3.

This PR attempts to add such support for explode for typescript-fetch.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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[bobvanderlinden]

I'm not sure whether this is a breaking change. It depends whether explode was used in the specification, but the mustache template ignored explode.

None of the petstore examples have changed, because none of the examples use explode: true. Note that the default for explode should be true, but it is currently false in openapi-generator, which I think is wrong. See https://swagger.io/docs/specification/serialization/#query

[bobvanderlinden]

@leonyu Could you have a look?

[leonyu]

Sorry, I no longer work for the company that used this. I don't really have the setup or context to develop for this. If it is a trivial PR, I can code review, that's about it. Beyond that, I can't contribute in any meaningful capacity.

[bobvanderlinden]

You should probably remove yourself from the technical committee. That said, who should I tag for this PR to review?

[leonyu]

I am not on the committee for this project. I never contributed directly to this project. My contributions were for the upstream at https://github.com/swagger-api/swagger-codegen .

My guess is you should probably ping one of the TypeScript person on the committee.

[bobvanderlinden]

Ah I wasn't aware. Sorry for the inconvenience.

Pinging technical committee of typescript:

@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02)

[amakhrov]

@bobvanderlinden could you please clarify why the existing logic that supports isCollectionFormatMulti doesn't work for you?

[bobvanderlinden]

You are totally right. It is already supported, but the default of explode is considered to be false by openapi-generator, whereas the standard describes the default to be true. That's probably where the confusion came from. Sorry for the PR and thanks for the clarification!

[jonnekleijer]

@amakhrov I am having a similar issue, I could not find (something similar to) isCollectionFormatMulti via the openapi-generator-cli: version openapi-generator-cli-3.3.4.jar?