Add unique_email field

db/migrate/20200310223351_add_unique_email_to_users.rb

@@ -0,0 +1,5 @@
+class AddUniqueEmailToUsers < ActiveRecord::Migration[5.2]
+  def change
+    add_column :users, :unique_email, :string
+  end
+end

db/migrate/20200310224203_add_index_unique_email_on_users.rb

@@ -0,0 +1,7 @@
+class AddIndexUniqueEmailOnUsers < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    add_index :users, :unique_email, algorithm: :concurrently
+  end
+end