Unexpected 'Fragment cannot be spread' error

[AlistairB]

Hi,

I am trying to use interfaces in my query using morpheus client. My gql file looks something like (excluded some excessive detail. Let me know if you want my full file):

"""
An object with an ID.
"""
interface Node {
  """
  ID of the object.
  """
  id: ID!
}

"""
Represents a Git object.
"""
interface GitObject {
  id: ID!
}

"""
Represents a Git reference.
"""
type Ref implements Node {
  id: ID!

  """
  The object the ref points to.
  """
  target: GitObject!
}

type Commit implements GitObject & Node {

  id: ID!

  """
  Authorship details of the commit.
  """
  author: String
}

type Tree implements GitObject & Node {
  """
  A list of tree entries.
  """
  treeName: String

  id: ID!
}

And my query looks something like:

defineByDocumentFile
  "../resource/minimal-github.graphql"
  [gql|
    query FetchRepoHeadCommit ($repoOwner: String!, $repoName: String!) {
      repository(name: $repoName, owner: $repoOwner) {
        defaultBranchRef {
          target {
            ... on Commit {
               author
            }
          }
        }
      }
    }
  |]

This fails with Fragment cannot be spread here as objects of type \"GitObject\" can never be of type \"Commit\. Changing target: GitObject! to target: Commit! makes it work. This confuses me as I have type Commit implements GitObject.

My expectation is that this should work based on 2 from https://github.com/morpheusgraphql/morpheus-graphql#interface . Am I missing something?

[nalchevanidze]

Thanks, you are right.

Query validation was written before we supported "interfaces", so i forgot to update it too.

I will fix it

[nalchevanidze]

@AlistairB i started it #495

[nalchevanidze]

@AlistairB please try it with actual master

[AlistairB]

Unfortunately I am still getting the same error building with morpheus based on commit 57de511 (current head on master).

[nalchevanidze]

ooh. that weird i wrote tests for it. can you share full schema?

[nalchevanidze]

and query with fragment

[AlistairB]

If you clone https://github.com/AlistairB/morpheus-repro and stack build it should reproduce the issue. It will only compile if you change https://github.com/AlistairB/morpheus-repro/blob/master/minimal-github.graphql#L89 to target: Commit!.

[nalchevanidze]

so, if we have schema.

interface GitObject {
  id: ID!
}

type Commit implements GitObject & Node {
  id: ID!
  author: String
}

type Query  {
  target: GitObject!
}

your query would look like:

{
   target {
    ... on Commit {
         author
         }
   }
}

and would be invalid. as long target guarantees to provide fields defined by GitObject (not by Commit).

so, if we had schema where target has concrete type Commit.

interface GitObject {
  id: ID!
}

type Commit implements GitObject & Node {
  id: ID!
  author: String
}

type Query  {
  target: Commit!
}

then we could query on it GitObject and Commit.

{
   t1: target {
    ... on Commit {
         author
         }
   }
   t2: target {
    ... on GitObject {
         author
         }
   }
}

[AlistairB]

Hmm, I'm not sure if I understand. My understanding is that you can use the spread operator to conditionally get data IF the type matches.

For example using https://developer.github.com/v4/explorer/

query { 
  repository(owner: "facebook", name: "react") {
    defaultBranchRef {
      target {
        id
        ... on Commit {
          author {
            name
          }
        }
      }
    }
  }
}

will fetch all this data, as target is in fact a Commit.

On the other hand:

query { 
  repository(owner: "facebook", name: "react") {
    defaultBranchRef {
      target {
        id
        ... on Blob {
          byteSize
        }
      }
    }
  }
}

Will only fetch the id, but will not return an error. This is because target: GitObject and type Blob implements GitObject, so it could be a Blob.

And you can combine them:

query { 
  repository(owner: "facebook", name: "react") {
    defaultBranchRef {
      target {
        id
        ... on Commit {
          author {
            name
          }
        }
        ... on Blob {
          byteSize
        }        
      }
    }
  }
}

In terms of the schema I am using, I have directly copied the bits I need from the official github public schema. See https://docs.github.com/en/graphql/overview/public-schema .

This query with the schema does work on github which makes me think it is valid?

[nalchevanidze]

@AlistairB okey. may i got something wrong. what you say looks like type casting.
i will read specifications and fix it.

[nalchevanidze]

@AlistairB can you check again , i fixed it #498

[nalchevanidze]

@AlistairB validation workded well bot there is some problem with type Generation that i am fixing in #499

[nalchevanidze]

@AlistairB now it should finally work

[AlistairB]

Thanks David! This is now working for most cases that I tested. The below is happy and works.

    query FetchRepoHeadCommit ($repoOwner: String!, $repoName: String!) {
      repository(name: $repoName, owner: $repoOwner) {
        defaultBranchRef {
          target {
            id
            ... on Commit {
              committedDate
            }
          }
        }
      }
    }

However, I get an error if I remove returning the id. eg.

    query FetchRepoHeadCommit ($repoOwner: String!, $repoName: String!) {
      repository(name: $repoName, owner: $repoOwner) {
        defaultBranchRef {
          target {
            ... on Commit {
              committedDate
            }
          }
        }
      }
    }

produces No Empty fields on \"RepositoryDefaultBranchRefTargetGitObject\". It seems that you must fetch something from the interface level otherwise it errors. I have pushed a new commit demonstrating this failure to https://github.com/AlistairB/morpheus-repro

Possibly worth noting that I believe putting interface level fields inside the spread is also valid. eg.

    query FetchRepoHeadCommit ($repoOwner: String!, $repoName: String!) {
      repository(name: $repoName, owner: $repoOwner) {
        defaultBranchRef {
          target {
            ... on Commit {
              id
              committedDate
            }
          }
        }
      }
    }

I think this is working aside from the above error. Although, I don't think supporting this style is super important.

[nalchevanidze]

thanks @AlistairB.

i fixed it in #500

[AlistairB]

Awesome, now working! thanks!