Fields on interface fragment not merged with implementing type fieldss

Issues GraphQL
1

What edition and version of Dgraph are you using?

Edition:

  • SlashGraphQL
  • [ x] Dgraph (community edition/Dgraph Cloud)

If you are using the community edition or enterprise edition of Dgraph, please list the version:
v21.03.0

Have you tried reproducing the issue with the latest release?

Yes

Steps to reproduce the issue (paste the query/schema if possible)

schema:

type LocalizedString {
   text: String!
   localization: String!
}

interface Product {
    id: ID!
    names: [LocalizedString !]!
}

type Computer implements Product {
    CPU: Int!
}

type Shoes implements Product {
    size: Int!
}

query:

{
   getComputer(id: "anyid") {
       names: {
            text
       }
       ... on Product {
              names: {
                 text
              }
       }
   }
}

Expected behaviour and actual result.

Since Computer implements Product, the query resolution should resolve the query as a single array of names in the same way as it resolves multiple scalar fields in a single field (

{
   getComputer(id: "anyid") {
       id
      id
   }
}

resolves correctly
)

instead Dgraph returns

"errors": [
    {
      "message": "Dgraph query failed because Dgraph execution failed because : while converting to subgraph: Computer.names not allowed multiple times in same sub-query.",
      "path": [
        "getComputer"
      ]
    }
  ],

This issue becomes quite annoying when using apollo or graphqltools to manipulate / stitch / federate the dgraph schema since the internal query planner of the gateway (being apollo or graphql tools) most of the time expands the interface fragment to the implementing type, creating queries where you have both the fragment of the interface and the one of the underlying type (which is GraphQL compliant) resulting in an error.