FIX: Add guard to prevent a primary UserEmail from being reassigned.

app/models/user_email.rb

@@ -12,6 +12,8 @@ class UserEmail < ActiveRecord::Base
   validates :email, email: true, format: { with: EmailValidator.email_regex },
                     if: :validate_email?
 
+  validate :user_id_not_changed, if: :primary
+
   validates :primary, uniqueness: { scope: [:user_id] }
 
   private
@@ -27,6 +29,14 @@ class UserEmail < ActiveRecord::Base
     return false if self.skip_validate_email
     email_changed?
   end
+
+  def user_id_not_changed
+    if self.will_save_change_to_user_id? && self.persisted?
+      self.errors.add(:user_id, I18n.t(
+        'active_record.errors.model.user_email.attributes.user_id.reassigning_primary_email')
+      )
+    end
+  end
 end
 
 # == Schema Information

config/locales/server.en.yml

@@ -403,6 +403,10 @@ en:
               unique_characters: "has too many repeated characters. Please use a more secure password."
             ip_address:
               signup_not_allowed: "Signup is not allowed from this account."
+        user_email:
+          attributes:
+            user_id:
+              reassigning_primary_email: "Reassigning a primary email to another user is not allowed."
         color_scheme_color:
           attributes:
             hex:

spec/models/user_spec.rb

@@ -37,6 +37,18 @@ describe User do
           expect(user.errors.messages).to include(:primary_email)
         end
       end
+
+      describe 'when primary_email is being reassigned to another user' do
+        it "should not be valid" do
+          user2 = Fabricate.build(:user, email: nil)
+          user.save!
+          user2.primary_email = user.primary_email
+
+          expect(user2).to_not be_valid
+          expect(user2.errors.messages).to include(:primary_email)
+          expect(user2.primary_email.errors.messages).to include(:user_id)
+        end
+      end
     end
   end