Handle unanswered question

app/controllers/merge_requests_controller.rb

@@ -10,6 +10,7 @@ class MergeRequestsController < ApplicationController
   ]
   before_action :authenticate_user!
   before_action :authenticate_scope!, except: [:create]
+  before_action :validate_response, only: %i[update]
 
   def absorbing_organisation; end
   def confirm_telephone_number; end
@@ -113,6 +114,13 @@ private
     merge_params
   end
 
+  def validate_response
+    if page == "absorbing_organisation" && merge_request_params[:absorbing_organisation_id].blank? && merge_request_params[:new_absorbing_organisation].blank?
+      @merge_request.errors.add(:absorbing_organisation_id, "Select an option")
+      render previous_template
+    end
+  end
+
   def merge_request_organisation_params
     {
       merge_request: @merge_request,

spec/requests/merge_requests_controller_spec.rb

@@ -260,6 +260,26 @@ RSpec.describe MergeRequestsController, type: :request do
         end
       end
 
+      context "when not answering the question" do
+        let(:merge_request) { MergeRequest.create!(requesting_organisation: organisation, absorbing_organisation: other_organisation) }
+        let(:params) do
+          { merge_request: { page: "absorbing_organisation" } }
+        end
+        let(:request) do
+          patch "/merge-request/#{merge_request.id}", headers:, params:
+        end
+
+        it "renders the error" do
+          request
+
+          expect(page).to have_content("Select an option")
+        end
+
+        it "does not update the request" do
+          expect { request }.not_to(change { merge_request.reload.attributes })
+        end
+      end
+
       context "when absorbing_organisation_id set to other" do
         let(:merge_request) { MergeRequest.create!(requesting_organisation: organisation, absorbing_organisation: other_organisation) }
         let(:params) do