AO3-6789 Return 404 when accessing challenge on nonexistent collection

app/controllers/challenge_signups_controller.rb

@@ -17,6 +17,8 @@ class ChallengeSignupsController < ApplicationController
   before_action :check_signup_in_collection, only: [:show, :edit, :update, :destroy, :confirm_delete]
 
   def load_challenge
+    raise ActiveRecord::RecordNotFound, "Couldn't find collection named '#{params[:collection_id]}'" unless @collection
+
     @challenge = @collection.challenge
     no_challenge and return unless @challenge
   end

spec/controllers/challenge_signups_controller_spec.rb

@@ -46,6 +46,25 @@
                                    "What challenge did you want to sign up for?")
       end
     end
+
+    context "when collection does not exist" do
+      it "raises a RecordNotFound error" do
+        fake_login
+        expect do
+          get :new, params: { collection_id: "nonexistent_collection" }
+        end.to raise_error ActiveRecord::RecordNotFound
+      end
+    end
+  end
+
+  describe "summary" do
+    context "when collection does not exist" do
+      it "raises a RecordNotFound error" do
+        expect do
+          get :summary, params: { collection_id: "nonexistent_collection" }
+        end.to raise_error ActiveRecord::RecordNotFound
+      end
+    end
   end
 
   describe "show" do