[TT-8776] UDG does not handle union as query result

[agata-wit]

Branch/Environment/Version

• Branch/Version: master
• Environment: all affected

Describe the bug
In case union is defined as the result of a UDG query operation with REST data sources, even though all data comes back from the upstream, the response doesn't resolve.
Described as well in community forum: https://community.tyk.io/t/using-union-on-tyk-udg-with-rest-datasource/6296

Reproduction steps
With the following schema:

type Query {
  getUser(id:Int):UserUnion
}
union UserUnion = User | CustomError


type CustomError{
  code: String
  message: String
}

type User{
  id:Int
  name:String
}

connecting a REST data source to getUser doesn't resolve correctly on response.
Response from upstream:

• correct from debug logs: /data (afterFetchHook.OnData): {“user”:{“id”:1,“name”:“Bob”,“status”:“PENDING”,“accessories”:[“A”,“B”,“C”]}}
• error from debug logs: /data (afterFetchHook.OnData): {“error”:{“code”:“ERR_01”,“message”:“some error”}}

Actual behavior
After running a query this is the response:

Expected behavior
Response should contain data parsed from the upstream.

[dennissetiawan]

There are also case where tyk does not forward the query correctly with GraphQL datasource. Check this reply https://community.tyk.io/t/using-union-on-tyk-udg-with-rest-datasource/6296/6?u=midnight

[agata-wit]

We will check if it's related or not. Thanks for the comment!