[BUG][Go] UnmarshalJson has unused variable err if spec has oneOf without discriminator

[code-lucidal58]

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

In the openAPI spec, if a schema has oneOf but does not have a discriminator and the config useOneOfDiscriminatorLookup is set to True, the unmarshalJson function is incorrectly generated. Such a scenario can arise when the schema looks as follows:

Pet:
  description: a pet
  oneOf:
  - type: 'null'
  - $ref: "#/components/schemas/dog"
  - $ref: "#/components/schemas/cat"

Considering the fact that there are other schemas in this spec that may have oneOf without type: 'null', the configuration useOneOfDiscriminatorLookup: True is required. As a result, the unmarshal function for the above object is as follows:

func (dst *Pet) UnmarshalJSON(data []byte) error {
	var err error
	// this object is nullable so check if the payload is null or empty string
	if string(data) == "" || string(data) == "{}" {
		return nil
	}
	return nil
}

Building this generates an error because the variable err is declared but not used.

openapi-generator version

Master build with last commit id 023e6dd

Suggest a fix

Allow unmarshalling of data even if nodiscriminator is mentioned whenuseOneOfDiscriminatorLookup: True

[wing328]

I've filed #8069 to fix the issue.

[code-lucidal58]

As per the change, if useOneOfDiscriminatorLookup is set to true, but the discriminator is missing, then the UnmarshalJSON function will be empty. Is this the desired result?

Expected: In the absence of discriminator, UnmarshalJSON can have the same code as when useOneOfDiscriminatorLookup is false.

openapi-generator/modules/openapi-generator/src/main/resources/go/model_oneof.mustache

Line 54 in 67c3d56

{{^useOneOfDiscriminatorLookup}}

[wing328]

Good point. I'll see what I can do.

[kandaaaaa]

Suggest to have this PR #11178