[typescript-fetch] Add (de)serializers for oneOfs

[eriktim]

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Using oneOfs with typescript-fetch results in union types, but lacks decoders and encoders for them. This PR patches that. I've also added support for the discriminator (resulting in tagged unions).

@wing328 Is there already a YAML available for testing oneOfs/allOfs/discriminators? Otherwise I would like to add a generic one.

cc @TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @nicokoenig @topce @akehir

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[wing328]

@eriktim thanks for the PR, which has been included in the v4.2.1 release: https://twitter.com/oas_generator/status/1195339336922759168

[Bessonov]

@eriktim I'm not sure that I understand how it supposed to work without a discriminator. I've some sort of MongoDb-Api description:

    DateSearch:
      oneOf:
      - type: object
        required:
        - $eq
        properties:
          $eq:
            type: string
            format: date
      - type: object
        required:
        - $ne
        properties:
          $ne:
            type: string
            format: date
[...]

The generated union type is correct:

export type DateSearch = DateSearchOneOf | DateSearchOneOf1 | [...]

But DateSearchToJSON seems to be wrong (and doesn't compile):

export function DateSearchToJSON(value?: DateSearch | null): any {
    if (value === undefined) {
        return undefined;
    }
    if (value === null) {
        return null;
    }
    return { ...DateSearchOneOfToJSON(value), ...DateSearchOneOf1ToJSON(value), [...] };
}

From logical point of view, the last line try to merge all properies, which isn't what for an union type should happens.

Technically, DateSearchOneOfToJSON expects DateSearchOneOf | null and therefore doesn't compile.

From my point of view, a check should be generated, which, in sense of oneOf, try to determine the type of DateSearch and return result of correct ToJSON transformer.

[Bessonov]

Ok, it's worser, I have no idea how to check in this case 🍡

[eriktim]

@Bessonov I currently do not have tsc at hand so I cannot reproduce your error (it would be nice to post that one as well). It looks like TypeScript does not take the spread operator as I had hoped. As stated in my description I would like to have this case tested as well, hence this PR should not have been merged in yet. But even if I would have had, using a oneOf without discriminator is not a great choice for TypeScript. Other languages are more capable of dealing with this trial-and-error decoding. Is there any reason you cannot use a discriminator?

[SimpleCookie]

Is this really resolved? I am still receiving the problem

@Data
@SuperBuilder(toBuilder = true)
@EqualsAndHashCode(callSuper = false)
@Schema(
    type = "object",
    oneOf = { First.class, Second.class }
)
public abstract class Foo {
    @Schema(required = true)
    private String type;
}

@Data
@EqualsAndHashCode(callSuper = true)
@SuperBuilder(toBuilder = true)
@Schema(type = "object")
@ApiModel(value = "Bar", parent = Foo.class)
public abstract class Bar extends Foo {
    @Schema(required = true)
    private String id;
}

error TS2305: Module '"."' has no exported member 'FooFromJSON'.
error TS2305: Module '"."' has no exported member 'FooToJSON'.

The example above might be slightly off, I cannot copy my code directly due to NDA.
Or am I doing something wrong?

Edit:
In my real code I have 2 classes extending Foo.

Edit 2:
In case anyone else have this issue..
I was using the OpenApi Generator version 4.1.1, switching to 4.2.3 solved the issue. So make sure you're using the latest version!