Using Fragment on an interface is not working properly

[akrejczinger]

Ariadne-codegen version: 0.11.0

I'm trying to define a fragment on the "Error" interface. In the actual code we have multiple different error classes implementing this same interface, here I have included a minimal example. I'm not allowed to share the actual code.
The query below works from the graphql UI but ariadne-codegen doesn't generate the right code.

Inputs that don't work

pyproject.toml configuration:

[tool.ariadne-codegen]
schema_path = "schema.graphql"
queries_path = "queries.graphql"
target_package_name = "gql_client"
async_client = false

Contents of schema.graphql:

interface Error {
  type: String
  title: String
  status: Int
  detail: String
  instance: String
}

type DeletionError implements Error {
  type: String
  title: String
  status: Int
  detail: String
  instance: String
}

type DeleteThingPayload {
  thing: Thing
  errors: [DeletionError!]
}

type Thing {
  id: ID!
  name: String
}

input DeleteThingInput {
  id: ID!
}

type Mutation {
  deleteThing(input: DeleteThingInput!): DeleteThingPayload!
}

Contents of queries.graphql:

fragment DefaultError on Error {
  ... on Error {
    type
    title
    status
    detail
    instance
  }
}

mutation MyDeleteThing($id: ID!) {
  deleteThing (input: {id: $id}) {
    thing {
      id
      name
    }
    errors {
      __typename
      ... DefaultError
    }
  }
}

When running ariadne-codegen, I expect to see all the error fields (type, title etc.) defined for the deleteThing query.
But instead only __typename is defined. Here's the relevant part of the generated code:

class MyDeleteThingDeleteThingErrors(BaseModel):
    typename__: Literal["DeletionError"] = Field(alias="__typename")

Inputs that work

Modifying the fragment to use the specific error type (DeletionError) works:

fragment DefaultError on DeletionError {
  ... on DeletionError {
    type
    title
    status
    detail
    instance
  }
}

Resulting code:

class MyDeleteThingDeleteThingErrors(BaseModel):
    typename__: Literal["DeletionError"] = Field(alias="__typename")
    type: Optional[str]
    title: Optional[str]
    status: Optional[int]
    detail: Optional[str]
    instance: Optional[str]

[bombsimon]

To me the changed working version looks like the way to do it but maybe I'm wrong given that it works in the UI. See the examples here f.ex. that has the title from the interface outside the spread and only spreads on concrete specific fields. Same here in the GraphQL docs where you only need spreading to get fields from the concrete types. So I usually do:

fragment <Fragment> on <Interface> {
  interfaceField
  anotherInterfaceField

  ... on <ConcreteType> {
    customFieldOnlyOnInterfaceImpl
  }

  ... on <AnotherConcreteType> {
    anotherCustomFieldOnlyOnAnotherInterfaceImpl
  }
}

By doing this it should still work in the UI and generate the proper code, right?

So imagining DeletionError had a specific field not on the interface called deleteFailReason I would write something like this and then just add more spreads for the other custom fields:

fragment DefaultError on Error {
  type
  title
  status
  detail
  instance

  ... on DeletionError {
    deleteFailReason
  }
}

[akrejczinger]

I'm actually new to GraphQL so it's possible I messed up something. I'll try your solution and see if it works.

[akrejczinger]

The modified DefaultError fragment works, thank you!