Merge pull request #2765 from govuk-forms/remove-n-queries-from-routes

Remove n+1 queries from multiple routes

Author: thomasiles

app/models/form.rb

@@ -224,6 +224,14 @@ def set_task_status_service(service)
     self.task_status_service = service
   end
 
+  # Return the next page or nil if there is no next page
+  # Use this when all pages are loaded to avoid N+1 queries,
+  # prefer Page.next_page for individual queries.
+  def next_page_after(current_page)
+    pair = pages.each_cons(2).find { |p, _next_p| p == current_page }
+    pair&.last
+  end
+
 private
 
   def set_external_id

app/services/routes/build_service.rb

@@ -26,14 +26,16 @@ def build_routes
   end
 
   def options_for_page(page)
-    return [] unless page.has_next_page?
+    next_page = form.next_page_after(page)
+
+    return [] unless next_page
 
     # Don't include the current page in the options
     options_without_page = all_goto_options.reject { |_, value| value == page.id }
 
     # Replace the next page with the default option
     options_without_page.map do |name, value|
-      if value == page.next_page
+      if value == next_page.id
         ["Go to question #{page.position.next}", Forms::RouteInput::DEFAULT_VALUE]
       else
         [name, value]

app/views/routes/show.html.erb

@@ -22,7 +22,7 @@
             row.with_value do
               capture do %>
               <p> <%= page.question_text %> </p>
-              <% if page.has_next_page? %>
+              <% if @routes_input.form.next_page_after(page).present? %>
                 <ul class="govuk-list">
                   <% routes.each do |route| %>
                     <li>

spec/models/form_spec.rb

@@ -1464,4 +1464,21 @@
       end
     end
   end
+
+  describe "#next_page_after" do
+    let(:pages) { [] }
+    let(:form) { build :form, pages: }
+
+    it "returns nil if there is no next page" do
+      expect(form.next_page_after(form.pages.first)).to be_nil
+    end
+
+    context "when there is a next page" do
+      let(:pages) { build_list :page, 5 }
+
+      it "returns the next page" do
+        expect(form.next_page_after(form.pages.second)).to eq(form.pages.third)
+      end
+    end
+  end
 end