fixes #3574

db/migrate/20180425194016_add_dmail_count_to_users.rb

@@ -0,0 +1,8 @@
+class AddDmailCountToUsers < ActiveRecord::Migration[5.1]
+  def change
+    ApplicationRecord.without_timeout do
+      add_column :users, :unread_dmail_count, :integer, default: 0, null: false
+      execute "update users set unread_dmail_count = (select count(*) from dmails where dmails.owner_id = users.id and dmails.is_read = false)"
+    end
+  end
+end