Improve UserEmail#email validation to use the index.

app/models/user_email.rb

@@ -7,14 +7,13 @@ class UserEmail < ActiveRecord::Base
 
   before_validation :strip_downcase_email
 
-  validates :email, presence: true, uniqueness: true
-
+  validates :email, presence: true
   validates :email, email: true, format: { with: EmailValidator.email_regex },
                     if: :validate_email?
 
+  validates :primary, uniqueness: { scope: [:user_id] }, if: :user_id
   validate :user_id_not_changed, if: :primary
-
-  validates :primary, uniqueness: { scope: [:user_id] }
+  validate :unique_email
 
   private
 
@@ -30,6 +29,12 @@ class UserEmail < ActiveRecord::Base
     email_changed?
   end
 
+  def unique_email
+    if self.will_save_change_to_email? && self.class.where("lower(email) = ?", email).exists?
+      self.errors.add(:email, :taken)
+    end
+  end
+
   def user_id_not_changed
     if self.will_save_change_to_user_id? && self.persisted?
       self.errors.add(:user_id, I18n.t(

spec/models/user_spec.rb

@@ -28,6 +28,19 @@ describe User do
         end
       end
 
+      describe 'when record has an email that as already been taken' do
+        it 'should not be valid' do
+          user2 = Fabricate(:user)
+          user.email = user2.email.upcase
+
+          expect(user).to_not be_valid
+
+          expect(user.errors.messages[:primary_email]).to include(I18n.t(
+            'activerecord.errors.messages.taken'
+          ))
+        end
+      end
+
       describe 'when user is staged' do
         it 'should still validate presence of primary_email' do
           user.staged = true