Fragments not working with interfaces

[letsar]

When I create a Fragment with a type which is an interface, or has fields which are interfaces, the generated code does not work at runtime.

For example let's have this schema:

interface Book {
  title: String!
  author: String!
}

type Textbook implements Book {
  title: String!
  author: String!
  courses: [String!]!
}

type ColoringBook implements Book {
  title: String!
  author: String!
  colors: [String!]!
}

type Query {
  books: [Book!]!
}

And this query:

fragment BookFragment on Book {
  title
  ... on Textbook {
      courses
  }
  ... on ColoringBook {
      colors
  }
}

query GetBooks {
  books {
    ...BookFragment
  }
}

These parameters are set on build.yaml:

  when_extensions:
    when: true
    maybeWhen: true

When we generate the code, we can see that nothing implements GBookFragment__asTextbook, yet we have this kind of generted code:

extension GBookFragmentWhenExtension on GBookFragment {
  _T when<_T>({
    required _T Function(GBookFragment__asTextbook) textbook,
    required _T Function(GBookFragment__asColoringBook) coloringBook,
    required _T Function() orElse,
  }) {
    switch (G__typename) {
      case 'Textbook':
        return textbook((this as GBookFragment__asTextbook));
      case 'ColoringBook':
        return coloringBook((this as GBookFragment__asColoringBook));
      default:
        return orElse();
    }
  }
}

Since nothing implements GBookFragment__asTextbook, the cast (this as GBookFragment__asTextbook) will fail.
Is there a workaround for that?

[knaeckeKami]

There's actually a deeper bug, the subtype-specific data does not even end up in the models

 final response = GGetBooksData.fromJson({
      "books": [
        {
          "__typename": "Textbook",
          "title": "Textbook",
          "courses": ["Math", "Science"]
        },
        {
          "__typename": "ColoringBook",
          "title": "ColoringBook",
          "colors": ["Red", "Blue"]
        }
      ]
    });

final GBookFragment textbook = response!.books[0]; // is of type _$GGetBooksData_books, with no data from the type conditions 

[letsar]

@knaeckeKami have you an idea on how we can fix this issue? Is it an issue from ferry or from another package?

[knaeckeKami]

It's an issue in the code generation in gql_code_builder. I didn't have time to look into this bug specifically yet. It might be an issue in the mergeSelections() or buildInlineFragmentClasses() function.

[knaeckeKami]

FWIW this seems to only happen when fragments are nested. So as a workaround you could try to flatten the query.

[btrautmann]

@nhannah Pretty sure this is the issue you were talking about 👀

[letsar]

Hi, I had some time to look into the issue. I don't know what is really causing the issue, but I came up with what should like the data.gql.dart file :
In the class GGetBooksData, instead of BuiltList<GGetBooksData_books> get books we should have BuiltList<GBookFragmentData> get books.
The class GGetBooksData_books does not seem to be useful in this case.
Now I need to see how to change this 😅

[knaeckeKami]

Cool!
I also looked a bit into it already.

I think there is something what with the mergeSelections function that doesn't handle nested fragments with type conditions properly

[nhannah]

@bestdan this is the issue we are seeing today