Add empty SDG goal show page

Note we're using the code instead of the ID to get the goal in the URL.
IMHO this is what most people would expect; visiting a URL with a "7"
takes you to SDG number 7, and not to the one with "7" as a database ID.

In order to avoid tests (either automated tests or manual tests) passing
by coincidence due to the goal ID and the goal code being the same, I'm
shuffling the codes before entering them in the databse.

I've tried using `resolve` in the routes so the code is automatically
taken into account, but it doesn't work since `resolve` cannot be used
inside a namespace, and here we're within the `sdg` namespace.

app/components/sdg/goals/index_component.html.erb

@@ -0,0 +1 @@
+<%= link_list(*goals.map { |goal| [goal.code_and_title, sdg_goal_path(goal.code)] }) %>

app/components/sdg/goals/index_component.rb

@@ -1,5 +1,6 @@
 class SDG::Goals::IndexComponent < ApplicationComponent
   attr_reader :goals
+  delegate :link_list, to: :helpers
 
   def initialize(goals)
     @goals = goals

app/components/sdg/goals/show_component.rb

@@ -0,0 +1,7 @@
+class SDG::Goals::ShowComponent < ApplicationComponent
+  attr_reader :goal
+
+  def initialize(goal)
+    @goal = goal
+  end
+end

app/controllers/sdg/goals_controller.rb

@@ -1,9 +1,12 @@
 class SDG::GoalsController < ApplicationController
   include FeatureFlags
   feature_flag :sdg
-  load_and_authorize_resource
+  load_and_authorize_resource find_by: :code, id_param: :code
 
   def index
     @goals = @goals.order(:code)
   end
+
+  def show
+  end
 end

app/views/sdg/goals/show.html.erb

@@ -0,0 +1 @@
+<%= render SDG::Goals::ShowComponent.new(@goal) %>

config/routes/sdg.rb

@@ -1,3 +1,3 @@
 namespace :sdg do
-  resources :goals, only: :index
+  resources :goals, param: :code, only: [:index, :show]
 end

db/sdg.rb

@@ -1,4 +1,4 @@
-(1..17).each do |code|
+(1..17).to_a.shuffle.each do |code|
   SDG::Goal.where(code: code).first_or_create!
 end
 

spec/routing/sdg_routes_spec.rb

@@ -0,0 +1,19 @@
+require "rails_helper"
+
+describe "SDG routes" do
+  it "maps goals to their code" do
+    expect(get("/sdg/goals/1")).to route_to(
+      controller: "sdg/goals",
+      action: "show",
+      code: "1"
+    )
+  end
+
+  it "requires using the code instead of the ID" do
+    expect(get(sdg_goal_path(SDG::Goal[2].code))).to route_to(
+      controller: "sdg/goals",
+      action: "show",
+      code: "2"
+    )
+  end
+end

spec/system/sdg/goals_spec.rb

@@ -21,4 +21,14 @@ describe "SDG Goals", :js do
       expect(page).to have_current_path sdg_goals_path
     end
   end
+
+  describe "Index" do
+    scenario "has links to SDGs" do
+      visit sdg_goals_path
+
+      click_link "7. Affordable and Clean Energy"
+
+      expect(page).to have_current_path sdg_goal_path(7)
+    end
+  end
 end