`link` does not support having no root mutation type

[kzlsakal]

Root Cause

The link macro adds the directive with root query and mutation types specified.

schema @link(url: "https:\/\/specs.apollo.dev\/federation\/v2.0", import: ["@key"]) {
  query: RootQueryType
  mutation: RootMutationType
}

If the schema does not define one of these types, the subgraph SDL becomes invalid. There is no validation to detect this compile-time.

Steps to reproduce

• Have a schema that does not have a root mutation field, such as:

defmodule InventoryWeb.Schema do
  use Absinthe.Schema
  use Absinthe.Federation.Schema

  link(url: "https://specs.apollo.dev/federation/v2.0", import: ["@key", "@extends", "@external"])

  query do
    extends()
  end

  object :product do
    key_fields("upc")
    extends()

    field :upc, non_null(:string) do
      external()
    end

    field(:in_stock, :boolean) do
      resolve(&resolve_in_stock_for_product/3)
    end
  end

  defp resolve_in_stock_for_product(%{upc: _upc} = _product, _args, _ctx), do: {:ok, true}
end

• Start the subgraph and the federated gateway.

Expected result

The gateway starts successfully

Actual result

The gateway returns the following error:

Loading gateway...
Error: A valid schema couldn't be composed. The following composition errors were found:
        [inventory] Cannot set schema mutation root to unknown type RootMutationType

[kzlsakal]

Note that extend schema introduced by absinthe 1.7.1 which was released a couple of days ago eliminates this issue.

extend schema do
  directive :link,
    url: "https://specs.apollo.dev/federation/v2.0",
    import: ["@key", "@extends", "@external"]
end

However, because it's macro-based, it won't support renaming individual directives like this:

extend schema do
  directive :link,
    url: "https://specs.apollo.dev/federation/v2.0",
    import: ["@key", %{name: "@override", as: "@replace"}]
end

That is because the map is converted to AST and not parsed in that macro.

Related: absinthe-graphql/absinthe#1225

[cschiewek]

@kzlsakal I'm trying to get this working, and I get Unknown directive link'.when I add the directive call to myextend schema` block. I'm using absinthe 1.7.6 and absinthe_federation 0.3.2. Any idea why that might be?

[kzlsakal]

@cschiewek can you double-check to ensure you have use Absinthe.Federation.Schema? If it's there, would you be able to post your schema file?

[kzlsakal]

This is now obsolete since extend schema method handles it by default. The test schemas and README were updated with examples in #87.