Graphql UnionType

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[ ] Regression 
[x?] Bug report
[ ] Feature request
[x] Documentation issue or request
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Current behavior

I try to integrate a UnionType with Graphql, but unfortunately without success.

I get the error message: Either the SearchResult type should provide a \"resolveType\" function or each possible type should provide an \"isTypeOf\" function."

Expected behavior

A working UnionType with Graphql 😃

Minimal reproduction of the problem with instructions

I have already tried to solve the problem with a ResolveProperty named resolveType inside of a resolver. But the function is not executed.

When I name the function __resolveType I get the error message Name "__resolveType" must not begin with "__", which is reserved by GraphQL introspection.

the createUnionType function:

export const SearchResultUnion = createUnionType({
    name: "SearchResult",  // the name of the GraphQL union
    types: [CPublicStore,CArticle, CService], // array of object types classes
});

My Search-ObjectType:

@ObjectType()
export class CSearch implements ISearch {
    @Field(type => [SearchResultUnion]) source: (CArticle | CService | CPublicStore)[]
    ...
}

and the resolver which should resolve the source property:

@Resolver(of => CSearch)
export class SearchTransform implements ResolverInterface<CSearch>{
...
    @ResolveProperty("__resolveType",()=>String)
    __resolveType(obj) {
       ....
    }
}

Environment

[System Information]
OS Version     : Linux 4.15
NodeJS Version : v10.15.1
YARN Version    : 1.12.3
[Nest Information]
platform-express version : 6.0.0
passport version         : 6.0.0
graphql version          : 6.0.2
swagger version          : 3.0.2
common version           : 6.0.0
core version             : 6.0.0

Thank you

[kamilmysliwiec]

It should be fixed in the @next version ($ npm i @nestjs/graphql@next). You will also have to update @nestjs/core to 6.2.0 ($ npm i @nestjs/core@6.2.0). Please, let me know if you face any issue.

[renehauck]

Okay, I found the problem. Is it possible that ResolveProperty does not work with a Union type? I have the following implementation:

@Resolver(of => CSearch)
export class SearchTransform implements ResolverInterface<CSearch>{
   ...
    @ResolveProperty("source",()=>[SearchResultUnion])
    async source(@Parent() recipe: CSearch) {
      ...
    }
}

but I get the following error: "message": "Abstract type SearchResult must resolve to an Object type at runtime for field CSearch.source with value [{}], received \"undefined\". Either the SearchResult type should provide a \"resolveType\" function or each possible type should provide an \"isTypeOf\" function."