Extending interfaces

Entity interfaces are similar to entities, but usually can't contribute new fields to the supergraph (see below for how to use @interfaceObject to extend interfaces).

Strawberry allows to define entity interfaces using the @strawberry.federation.interface decorator, here's an example:

import strawberry


@strawberry.federation.interface(keys=["id"])
class Media:
    id: strawberry.ID

This will generate the following GraphQL type:

type Media @key(fields: "id") @interface {
  id: ID!
}

Extending Entity interfaces (Apollo Federation)

In federation you can use @interfaceObject to extend interfaces from other services. This is useful when you want to add fields to an interface that is implemented by types in other services.

Entity interfaces that extend other interfaces are defined by annotating an interface with the @strawberry.federation.interface_object decorator, like in example below:

import strawberry


@strawberry.federation.interface_object(keys=["id"])
class Media:
    id: strawberry.ID
    title: str

This will generate the following GraphQL type:

type Media @key(fields: "id") @interfaceObject {
  id: ID!
  title: String!
}

@strawberry.federation.interface_object is necessary because if we were to extend the Media interface using @strawberry.federation.interface, we'd need to also define all the types implementing the interface, which will make the schema hard to maintain (every updated to the interface and types implementing it would need to be reflected in all subgraphs declaring it).

Resolving references

Entity interfaces are also used to resolve references to entities. The same rules as entities apply here. Here's a basic example:

import strawberry


@strawberry.federation.interface_object(keys=["id"])
class Media:
    id: strawberry.ID
    title: str

    # TODO: check this

    @classmethod
    def resolve_reference(cls, id: strawberry.ID) -> "Media":
        # here we could fetch the media from the database
        # or even from an API
        return Media(id=id, title="My Media")