Ensure date of birth is suitable

A business requirement is to only allow applicants between 21 and 80
years old, as per the previous IRP application.

app/forms/journeys/get_a_teacher_relocation_payment/personal_details_form.rb

@@ -0,0 +1,19 @@
+module Journeys
+  module GetATeacherRelocationPayment
+    class PersonalDetailsForm < PersonalDetailsForm
+      with_options if: -> { date_of_birth && date_of_birth.is_a?(Date) } do
+        validates :date_of_birth,
+          comparison: {
+            less_than_or_equal_to: ->(_) { 21.years.ago },
+            message: PersonalDetailsForm.i18n_error_message(:below_min_age)
+          }
+
+        validates :date_of_birth,
+          comparison: {
+            greater_than: ->(_) { 80.years.ago },
+            message: PersonalDetailsForm.i18n_error_message(:over_max_age)
+          }
+      end
+    end
+  end
+end

app/models/journeys/get_a_teacher_relocation_payment.rb

@@ -18,7 +18,8 @@ module GetATeacherRelocationPayment
         "entry-date" => EntryDateForm,
         "nationality" => NationalityForm,
         "passport-number" => PassportNumberForm,
-        "headteacher-details" => HeadteacherDetailsForm
+        "headteacher-details" => HeadteacherDetailsForm,
+        "personal-details" => PersonalDetailsForm
       }
     }
   end

config/locales/en.yml

@@ -770,6 +770,10 @@ en:
           school_headteacher_name: "Enter the name of the headteacher of the school where you are employed as a teacher"
         errors:
           school_headteacher_name: "Enter the headteacher's name"
+      personal_details:
+        errors:
+          over_max_age: Age must be below 80
+          below_min_age: Age must be above 21
 
     check_your_answers:
       part_one:

spec/forms/journeys/get_a_teacher_relocation_payment/personal_details_form_spec.rb

@@ -0,0 +1,49 @@
+require "rails_helper"
+
+RSpec.describe Journeys::GetATeacherRelocationPayment::PersonalDetailsForm, type: :model do
+  let(:journey_session) { build(:get_a_teacher_relocation_payment_session) }
+
+  subject(:form) do
+    described_class.new(
+      journey_session: journey_session,
+      journey: Journeys::GetATeacherRelocationPayment,
+      params: ActionController::Parameters.new({})
+    )
+  end
+
+  describe "validations" do
+    before do
+      form.assign_attributes(
+        day: date_of_birth.day,
+        month: date_of_birth.month,
+        year: date_of_birth.year
+      )
+
+      form.valid?
+    end
+
+    context "with too young of a birth date" do
+      let(:date_of_birth) { 20.years.ago }
+
+      it "does not permit the date of birth" do
+        expect(form.errors[:date_of_birth]).to include("Age must be above 21")
+      end
+    end
+
+    context "with too old of a birth date" do
+      let(:date_of_birth) { 80.years.ago }
+
+      it "does not permit the date of birth" do
+        expect(form.errors[:date_of_birth]).to include("Age must be below 80")
+      end
+    end
+
+    context "with a valid birth date" do
+      let(:date_of_birth) { 30.years.ago }
+
+      it "permits the date of birth" do
+        expect(form.errors[:date_of_birth]).to be_empty
+      end
+    end
+  end
+end