Unused fragment error in the subqueries generated for a union fragment

[eole1712]

Hello

Describe the bug

When we use a fragment in a query, in the subquery that the router builds the fragment is replaced by the selected keys and is removed from the query.

However, if the fragment refers to one of the options of a union, the router does not remove the fragment from the query after replacing it, which triggers a GRAPHQL_VALIDATION_FAILED error in the subgraph because of an unused fragment.

To Reproduce

1. Setup a service with the followed schema

type Employee {
   job_name: String!
}

type Student {
   school_name: String!
}

union UserType = Student | Employee

type User {
  id: Int!
  type: UserType!
}

type Query {
  user: User
}

2. Submit request

fragment employeeFragment on Employe {
   __typename
   job_name
}

fragment userFragment on User {
  __typename
  id
  type {
    ...employeeFragment
    __typename
  }
}

query test {
  user {
    __typename
    ...userFragment
  }
}

3. See error

The generated subgraph request is supposed to be :

query test__servicea__0 {
  user {
    __typename
    id
    type {
      __typename
      ... on Employee {
        job_name
      }
    }
  }
}

But instead the query generated for the subgraph is :

query test__servicea__0 {
  user {
    __typename
    id
    type {
      __typename
      ... on Employee {
        job_name
      }
    }
  }
}

fragment employeeFragment on Employe {
   __typename
   job_name
}

This generated the error GRAPHQL_VALIDATION_FAILED on the subgraph with the message : Fragment "employeeFragment" is never used.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Also sorry about the title of the issue, I had a hard time putting it into a sentence.

[abernix]

Thanks for reporting this! This is a duplicate of apollographql/federation#2092 and it'll be fixed in an upcoming release that brings in the updated query planner from that other repository. You can track #1546 for the issue that will eventually bring it in.