Use GraphQL for rendering World Index page

This will allow us to build a proof of concept for using GraphQL on
GOV.UK.

It will run alongside the existing retrieval from Content Store to allow
us to switch the rendering used in different environment, through a
feature flag or A-B test.

The feature is switched on by adding a `GRAPHQL_FEATURE_FLAG`
environment variable.

app/controllers/world_controller.rb

@@ -1,7 +1,13 @@
 class WorldController < ApplicationController
   def index
-    index = WorldIndex.find!("/world")
-    @presented_index = WorldIndexPresenter.new(index)
+    if ENV["GRAPHQL_FEATURE_FLAG"]
+      index = WorldIndexGraphql.find!("/world")
+      @presented_index = WorldIndexPresenterGraphql.new(index)
+    else
+      index = WorldIndex.find!("/world")
+      @presented_index = WorldIndexPresenter.new(index)
+    end
+
     setup_content_item_and_navigation_helpers(index)
   end
 end

app/models/world_index_graphql.rb

@@ -0,0 +1,14 @@
+class WorldIndexGraphql
+  attr_reader :content_item
+
+  def initialize(content_item)
+    @content_item = content_item
+  end
+
+  def self.find!(base_path)
+    query = Graphql::WorldIndexQuery.new(base_path).query
+
+    edition = Services.publishing_api.graphql_query(query).dig("data", "edition")
+    new(edition)
+  end
+end

app/presenters/world_index_presenter_graphql.rb

@@ -0,0 +1,15 @@
+class WorldIndexPresenterGraphql < WorldIndexPresenter
+  def title
+    @world_index.content_item["title"]
+  end
+
+  def international_delegations
+    @world_index.content_item["internationalDelegations"]
+  end
+
+private
+
+  def world_locations
+    @world_index.content_item["worldLocations"]
+  end
+end

app/queries/graphql/world_index_query.rb

@@ -0,0 +1,31 @@
+class Graphql::WorldIndexQuery
+  def initialize(base_path)
+    @base_path = base_path
+  end
+
+  def query
+    <<-QUERY
+      fragment worldLocationInfo on WorldLocation {
+        active
+        name
+        slug
+      }
+
+      {
+        edition(basePath: "#{@base_path}") {
+          ... on WorldIndex {
+            title
+
+            worldLocations {
+              ...worldLocationInfo
+            }
+
+            internationalDelegations {
+              ...worldLocationInfo
+            }
+          }
+        }
+      }
+    QUERY
+  end
+end

spec/controllers/world_controller_spec.rb

@@ -1,12 +1,51 @@
 RSpec.describe WorldController do
   describe "GET index" do
-    before do
-      stub_content_store_has_item("/world", GovukSchemas::Example.find("world_index", example_name: "world_index"))
+    context "without the GraphQL feature flag" do
+      before do
+        stub_content_store_has_item("/world", GovukSchemas::Example.find("world_index", example_name: "world_index"))
+      end
+
+      it "has a success response" do
+        get :index
+        expect(response).to have_http_status(:success)
+      end
     end
 
-    it "has a success response" do
-      get :index
-      expect(response).to have_http_status(:success)
+    context "with the GraphQL feature flag" do
+      before do
+        stub_const("ENV", { "GRAPHQL_FEATURE_FLAG" => true })
+
+        graphql_query = Graphql::WorldIndexQuery.new("/world").query
+
+        graphql_response = {
+          "data": {
+            "edition": {
+              "title": "Help and services around the world",
+              "worldLocations": [
+                {
+                  "active": true,
+                  "name": "Test World Location",
+                  "slug": "test-world-location",
+                },
+              ],
+              "internationalDelegations": [
+                {
+                  "active": false,
+                  "name": "Test International Delegation",
+                  "slug": "test-international-delegation",
+                },
+              ],
+            },
+          },
+        }
+
+        stub_publishing_api_graphql_query(graphql_query, graphql_response)
+      end
+
+      it "has a success response" do
+        get :index
+        expect(response).to have_http_status(:success)
+      end
     end
   end
 end