feat: support @key directive resolvable argument (#249)

* Add resolvable argument to key annotation

This adds support for federation 2.0's new [resolvable
field](https://www.apollographql.com/docs/federation/federation-2/new-in-federation-2/#changes-to-key).

* Add snippets about resolvable: false

* Linter fix

* Add test for unresolvable @key directive

---------

Co-authored-by: Robin Goos <[email protected]>

Author: jturkel

README.md

@@ -178,6 +178,14 @@ class User < BaseObject
 end
 ```
 
+As well as non-resolvable keys:
+
+```ruby
+class User < BaseObject
+  key fields: :id, resolvable: false
+end
+```
+
 See [field set syntax](#field-set-syntax) for more details on the format of the `fields` option.
 
 ### The `@external` directive

lib/apollo-federation/object.rb

@@ -32,13 +32,15 @@ def tag(name:)
         add_directive(name: 'tag', arguments: [name: 'name', values: name])
       end
 
-      def key(fields:, camelize: true)
+      def key(fields:, camelize: true, resolvable: true)
+        arguments = [
+          name: 'fields',
+          values: ApolloFederation::FieldSetSerializer.serialize(fields, camelize: camelize),
+        ]
+        arguments.append(name: 'resolvable', values: resolvable) unless resolvable
         add_directive(
           name: 'key',
-          arguments: [
-            name: 'fields',
-            values: ApolloFederation::FieldSetSerializer.serialize(fields, camelize: camelize),
-          ],
+          arguments: arguments,
         )
       end
     end

spec/apollo-federation/service_field_v2_spec.rb

@@ -1326,6 +1326,56 @@ def execute_sdl(schema)
       )
     end
 
+    it 'returns valid SDL for unresolvable @key directives' do
+      product = Class.new(base_object) do
+        graphql_name 'Product'
+        key fields: :upc, resolvable: false
+
+        field :upc, String, null: false
+      end
+
+      schema = Class.new(base_schema) do
+        orphan_types product
+        federation version: '2.0'
+      end
+
+      expect(execute_sdl(schema)).to match_sdl(
+        <<~GRAPHQL,
+          extend schema
+            @link(url: "https://specs.apollo.dev/federation/v2.3", import: ["@inaccessible", "@tag"])
+
+          type Product @federation__key(fields: "upc", resolvable: false) {
+            upc: String!
+          }
+        GRAPHQL
+      )
+    end
+
+    it 'returns valid SDL for resolvable @key directives' do
+      product = Class.new(base_object) do
+        graphql_name 'Product'
+        key fields: :upc, resolvable: true
+
+        field :upc, String, null: false
+      end
+
+      schema = Class.new(base_schema) do
+        orphan_types product
+        federation version: '2.0'
+      end
+
+      expect(execute_sdl(schema)).to match_sdl(
+        <<~GRAPHQL,
+          extend schema
+            @link(url: "https://specs.apollo.dev/federation/v2.3", import: ["@inaccessible", "@tag"])
+
+          type Product @federation__key(fields: "upc") {
+            upc: String!
+          }
+        GRAPHQL
+      )
+    end
+
     it 'returns valid SDL for @external directives' do
       product = Class.new(base_object) do
         graphql_name 'Product'