Enforce unique contact email for group hierarchy

Changelog: added

app/models/customer_relations/contact.rb

@@ -24,6 +24,7 @@ class CustomerRelations::Contact < ApplicationRecord
   validates :email, length: { maximum: 255 }
   validates :description, length: { maximum: 1024 }
   validate :validate_email_format
+  validate :unique_email_for_group_hierarchy
 
   def self.find_ids_by_emails(group_id, emails)
     raise ArgumentError, "Cannot lookup more than #{MAX_PLUCK} emails" if emails.length > MAX_PLUCK
@@ -39,4 +40,14 @@ def validate_email_format
 
     self.errors.add(:email, I18n.t(:invalid, scope: 'valid_email.validations.email')) unless ValidateEmail.valid?(self.email)
   end
+
+  def unique_email_for_group_hierarchy
+    return unless group
+    return unless email
+
+    duplicate_email_exists = CustomerRelations::Contact
+      .where(group_id: group.self_and_hierarchy.pluck(:id), email: email)
+      .where.not(id: id).exists?
+    self.errors.add(:email, _('contact with same email already exists in group hierarchy')) if duplicate_email_exists
+  end
 end

locale/gitlab.pot

@@ -41910,6 +41910,9 @@ msgstr ""
 msgid "compliance violation has already been recorded"
 msgstr ""
+msgid "contact with same email already exists in group hierarchy"
+msgstr ""
+
 msgid "container_name can contain only lowercase letters, digits, '-', and '.' and must start and end with an alphanumeric character"
 msgstr ""

spec/models/customer_relations/contact_spec.rb

@@ -26,6 +26,38 @@
     it_behaves_like 'an object with RFC3696 compliant email-formatted attributes', :email
   end
 
+  describe '#unique_email_for_group_hierarchy' do
+    let_it_be(:parent) { create(:group) }
+    let_it_be(:group) { create(:group, parent: parent) }
+    let_it_be(:subgroup) { create(:group, parent: group) }
+
+    let_it_be(:existing_contact) { create(:contact, group: group) }
+
+    context 'with unique email for group hierarchy' do
+      subject { build(:contact, group: group) }
+
+      it { is_expected.to be_valid }
+    end
+
+    context 'with duplicate email in group' do
+      subject { build(:contact, email: existing_contact.email, group: group) }
+
+      it { is_expected.to be_invalid }
+    end
+
+    context 'with duplicate email in parent group' do
+      subject { build(:contact, email: existing_contact.email, group: subgroup) }
+
+      it { is_expected.to be_invalid }
+    end
+
+    context 'with duplicate email in subgroup' do
+      subject { build(:contact, email: existing_contact.email, group: parent) }
+
+      it { is_expected.to be_invalid }
+    end
+  end
+
   describe '#before_validation' do
     it 'strips leading and trailing whitespace' do
       contact = described_class.new(first_name: '  First  ', last_name: ' Last  ', phone: '  123456 ')