Custom attributes using serialization

edit.html.erb

<% form_for(@user) do |f| %>
  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>

  <% f.fields_for :custom, @user.custom do |custom_form| %>
    <% custom_form.object.marshal_dump.each_key do |custom_field| %>
      <p>
        <%= custom_form.label custom_field %><br />
        <%= custom_form.text_field custom_field %>
      </p>
    <% end %>
  <% end %>
  <p>
    <%= f.submit 'Update' %>
  </p>
<% end %>

user.rb

require "ostruct"

class User < ActiveRecord::Base
  serialize :custom

  def after_initialize
    self.custom = OpenStruct.new(custom)
  end

  before_validation :convert_custom_attributes_to_hash
  after_validation :convert_custom_attributes_to_ostruct

  def convert_custom_attributes_to_hash
    self.custom = custom.marshal_dump if custom.kind_of? OpenStruct
  end

  def convert_custom_attributes_to_ostruct
    self.custom = OpenStruct.new(custom) if custom.kind_of? Hash
  end
end