[go-gin-server] Add nullable support

[alex-korobko]

Description

It seems like nullable is not supported yet for primitive types in model generation for Go . I checked for go-gin-server generation but it seems to be language wide issue.

openapi-generator version

openapi-generator version 3.3.4

OpenAPI declaration file content or url

Here is the simple model to check
https://editor.swagger.io/?url=https://gist.github.com/alex-korobko/8d3f4fbe817d4f1420ab779c91dddf94

Command line used for generation

openapi-generator generate --generator-name go-gin-server -i $SPEC -o $AUTO_CODE_DIR

Steps to reproduce

Use spec from the gist file (the link is above in the description).
Try to generate models for generator go-gin-server

Related issues/PRs

None found so far neither opened nor closed.

Suggest a fix/enhancement

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[alex-korobko]

BTW when openapi-generator generates references, it uses pointers.
For example, for the spec

    Product:
      type: object
      properties:
        ProductDimensions: # only provided if ProductDimensions was specified in the Product request
          $ref: '#/components/schemas/ProductDimensions'

the generated model contains pointer to the ProductDimensions struct

package openapi

type Product struct {
	ProductDimensions *ProductDimensions `json:"ProductDimensions,omitempty"`

[ackintosh]

@alex-korobko Thanks for reporting this issue!

cc Technical Committee: @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09

[alex-korobko]

Thanks for the attention to the issue, @ackintosh ! I'd be glad to help to fix the issue, if you guys provide some initial guidance.

[ackintosh]

@alex-korobko Thanks! There are some PRs helps you as a reference which adds nullable support to the generator. 💡 :

• Server generator
  • rust-server : [rust-server] Restore support for nullable #936
• Client generator
  • typescript-axios : Add nullable support to typescript-axios #1743
  • Typescript-Angular : [Typescript-Angular] add nullable support #1730
  • Perl : Add "nullable" support to Perl operation's parameters #1690

You may use the mustache tag {{#isNullable}}...{{/isNullable}} {{^isNullable}}...{{/isNullable}} to achieve the nullable feature. ( templates of go-gin-server is here ) 📝

Please let us know if you need help. 😉

[alex-korobko]

Created PR with updated mustache files for all types of Go lang generators.
#1869