downcase all GpgSignature#gpg_key_user_email

db/post_migrate/20170828124518_downcase_gpg_key_user_email_on_gpg_signatures.rb

+class DowncaseGpgKeyUserEmailOnGpgSignatures < ActiveRecord::Migration
+  DOWNTIME = false
+
+  include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
+  class GpgSignature < ActiveRecord::Base
+    self.table_name = 'gpg_signatures'
+
+    include EachBatch
+  end
+
+  def up
+    GpgSignature.each_batch do |relation|
+      relation.update_all('gpg_key_user_email = LOWER(gpg_key_user_email)')
+    end
+  end
+
+  def down
+    # we can't revert the downcasing, but actually we don't need to really, as
+    # downcasing the emails is not a harmful change.
+  end
+end