[BUG] [Go] non-required field of structs incorrectly generated

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

There's a quirk in go when serializing the generated model classes.

Take a look at the petstore's Pet schema object, the field category here.

category is not listed as required. So the generated code looks like:

type Pet struct {
	// ...
	Category Category `json:"category,omitempty"`
	// ...
}

So we expect the field to be absent, when it's not explicitly defined.

This does not happen, though. It's always present. The reason is this.

The link summarized: putting omitempty on a field that is a struct and not a base type, will always generate the struct, because a struct doesn't have a default value that could be checked.

This is actually a real bummer, following good API design practices ALL fields should be optional. So this bug now surfaces in our whole API. But only on server-side. E.g. the typescript generation doesn't have this problem.

openapi-generator version

5.1.1

OpenAPI declaration file content or url

petstore example

Generation Details

In the root folder of this project:

docker run -v $(pwd):/workspace docker.io/openapitools/openapi-generator-cli:latest generate -g go-server -i  /workspace/modules/openapi-generator/src/test/resources/3_0/petstore.yaml -o /workspace/tmp

Steps to reproduce

Generate the example as outlined above.

Related issues/PRs

Couldn't find one.

Suggest a fix

Solution: for non-required fields generate a pointer to the struct:

type Pet struct {
	// ...
	Category *Category `json:"category,omitempty"`
	// ...
}

Workaround: define the struct as nullable:

    Category:
      title: Pet category
      description: A category for a pet
      type: object
      nullable: true

However that's not nice at all.
I need to define the nullable on the Category, not where it's actually used (because the Category is $ref'd and not defined inline). So now all usages of category become nullable.

That would be okay...but way more importantly, now the generation for other languages has this nullable + not required twice. E.g. typescript will be generated as category?: Category | null

Start with modules/openapi-generator/src/test/resources/3_0/petstore.yaml and generate the go-server output using the Docker command in the issue. Inspect the generated Pet model, especially the non-required category field. Done means non-required struct fields are omitted when undefined without making unrelated nullable changes, and the generated output demonstrates that behavior.