feat: add validation

app/models/validations/sales/household_validations.rb

@@ -6,6 +6,7 @@ module Validations::Sales::HouseholdValidations
       validate_person_age_matches_relationship(record, n)
       validate_person_age_and_relationship_matches_economic_status(record, n)
       validate_person_age_matches_economic_status(record, n)
+      validate_child_12_years_younger(record, n)
     end
     shared_validate_partner_count(record, 6)
   end
@@ -79,6 +80,19 @@ private
     end
   end
 
+  def validate_child_12_years_younger(record, person_num)
+    buyer_1_age = record.public_send("age1")
+    person_age = record.public_send("age#{person_num}")
+    relationship = record.public_send("relat#{person_num}")
+    return unless buyer_1_age && person_age && relationship
+
+    if person_age > buyer_1_age - 12 && person_is_child?(relationship)
+      record.errors.add "age1", I18n.t("validations.household.age.child_12_years_younger")
+      record.errors.add "age#{person_num}", I18n.t("validations.household.age.child_12_years_younger")
+      record.errors.add "relat#{person_num}", I18n.t("validations.household.relat.child_12_years_younger")
+    end
+  end
+
   def person_is_partner?(relationship)
     relationship == "P"
   end

config/locales/en.yml

@@ -341,6 +341,7 @@ en:
         child_under_16: "Answer cannot be under 16 as person’s %{person_num} working situation is not ‘child under 16’"
         child_over_16: "Answer cannot be over 16 as person’s %{person_num} working situation is ‘child under 16‘"
         child_over_20: "Answer cannot be 20 or over as the relationship is ‘child’"
+        child_12_years_younger: "A child must be at least 12 years younger than their parent"
         not_student_16_19: "Answer cannot be between 16 and 19 as person %{person_num} is a child of the lead tenant but is not a full-time student"
         student_16_19:
           cannot_be_16_19: