Composition error when there are no types for union _Entity

[kzlsakal]

I came across this error while testing @apollo/gateway 2.0 with federated subgraphs and using mix task absinthe.federation.schema.sdl.

If we use Absinthe.Federation.Notation on a schema that does not introduce an extendable type, or does not extend a type from another subgraph, there are no types for union _Entity and we get the following composition error from the gateway:

[service_a] Unknown type "_Entity".  
[service_a] Union type _Entity must define one or more member types. 

Examples

The following schema will result in composition error because there are no types for union _Entity:

defmodule SchemaWithoutExtendedTypes do
  use Absinthe.Schema
  use Absinthe.Federation.Schema

  query do
    field :test, :string
  end
end

The following schema will do just fine:

defmodule SchemaWithExtendableType do
  use Absinthe.Schema
  use Absinthe.Federation.Schema

  query do
    field :foo, :bar
  end

  object :bar do
    key_fields("id")

    field :id, :string
  end
end

The following schema will have a successful subraph composition. The supergraph composition on the gateway level will only fail if there are no other services providing this type as an extendable type, which is expected.

defmodule SchemaWithExtendedTypeFromAnotherSubgraph do
  use Absinthe.Schema
  use Absinthe.Federation.Schema

  query do
    field :foo, :bar
  end

  object :bar do
    key_fields("id")
    extends()

    field :id, non_null(:id), do: external()
  end
end