add: invite code

app/models/concerns/dcodes.rb

@@ -0,0 +1,13 @@
+module Dcodes
+  DCODES = %W(1 2 3 4 5 6 7 8 9 a b c f e f g h i j k l m n o p q r s t u v w x y z)
+
+  extend ActiveSupport::Concern
+
+  def generate_dcode(field, num, pre='')
+    code = DCODES.sample(num).join
+    while self.class.exists?("#{field}": pre+code) do
+      code = DCODES.sample(num).join
+    end
+    code
+  end
+end