Restore invalid postcode input value in postcode field

app/controllers/form_controller.rb

@@ -93,6 +93,7 @@ class FormController < ApplicationController
           @questions = request.params["related_question_ids"].map { |id| @log.form.get_question(id, @log) }
           render "form/check_errors"
         else
+          restore_placeholder_values(@log, @page)
           if flash[:errors].present?
             restore_previous_errors(flash[:errors])
             restore_error_field_values(flash[:log_data])
@@ -125,6 +126,14 @@ private
     @log.assign_attributes(previous_responses_to_reset)
   end
 
+  def restore_placeholder_values(log, page)
+    page.questions.each do |question|
+      next if question.placeholder_method.blank?
+
+      log[question.id] ||= log.send(question.placeholder_method)
+    end
+  end
+
   def restore_previous_errors(previous_errors)
     return unless previous_errors
 

app/models/form/lettings/questions/postcode_for_full_address.rb

@@ -20,6 +20,7 @@ class Form::Lettings::Questions::PostcodeForFullAddress < ::Form::Question
     @disable_clearing_if_not_routed_or_dynamic_answer_options = true
     @question_number = QUESTION_NUMBER_FROM_YEAR[form.start_date.year] || QUESTION_NUMBER_FROM_YEAR[QUESTION_NUMBER_FROM_YEAR.keys.max]
     @hide_question_number_on_page = true
+    @placeholder_method = "postcode_full_input_placeholder"
   end
 
   QUESTION_NUMBER_FROM_YEAR = { 2023 => 12, 2024 => 13 }.freeze

app/models/form/question.rb

@@ -8,7 +8,7 @@ class Form::Question
                 :top_guidance_partial, :bottom_guidance_partial, :prefix, :suffix,
                 :requires_js, :fields_added, :derived, :check_answers_card_number,
                 :unresolved_hint_text, :question_number, :hide_question_number_on_page,
-                :plain_label, :error_label
+                :plain_label, :error_label, :placeholder_method
 
   def initialize(id, hsh, page)
     @id = id

app/models/log.rb

@@ -294,6 +294,10 @@ class Log < ApplicationRecord
     !!public_send("age#{person_num}_known")&.zero?
   end
 
+  def postcode_full_input_placeholder
+    postcode_full_input
+  end
+
 private
 
   # Handle logs that are older than previous collection start date

spec/requests/form_controller_spec.rb

@@ -371,8 +371,10 @@ RSpec.describe FormController, type: :request do
     end
 
     describe "GET" do
+      let(:current_time) { Time.zone.local(2022, 5, 1) }
+
       around do |example|
-        Timecop.freeze(Time.zone.local(2022, 5, 1)) do
+        Timecop.freeze(current_time) do
           Singleton.__init__(FormHandler)
           example.run
         end
@@ -443,6 +445,21 @@ RSpec.describe FormController, type: :request do
             end
           end
         end
+
+        context "when viewing the manual adress entry page" do
+          let(:current_time) { Time.zone.local(2024, 5, 1) }
+
+          context "and postcode input exists but postcode is not set" do
+            let(:user) { create(:user, :support) }
+            let(:lettings_log) { create(:lettings_log, :setup_completed, postcode_full_input: "A1", address_line1_input: "xx", postcode_full: nil, uprn_known: 0, uprn_selection: "uprn_not_listed") }
+
+            it "displays the postcode_full_input in postcode_full field" do
+              get "/lettings-logs/#{lettings_log.id}/address", headers: headers, params: {}
+              expect(page).to have_field("lettings-log-postcode-full-field", with: "A1")
+              expect(lettings_log.reload.postcode_full).to eq(nil)
+            end
+          end
+        end
       end
 
       context "when displaying check answers pages" do