filter_json.rb

module FilterJson
  extend ActiveSupport::Concern

  module ClassMethods
    def filter_json (*attributes)
      (@filter_attributes ||= Set.new).merge(attributes.map { |a| a.to_s })
    end

    def filter_attributes
      if superclass.respond_to?(:filter_attributes)
        superclass.filter_attributes + @filter_attributes
      else
        @filter_attributes
      end
    end
  end

  def as_json (options = nil)
    options ||= {}
    super(options.deep_merge({:except => self.class.filter_attributes.to_a}))
  end
end

secure_password.rb

module SecurePassword
  extend  ActiveSupport::Concern
  include FilterJson

  included do
    has_secure_password :validations => false
    filter_json :password_digest
    #...
  end
#...
end

user.rb

class User < ActiveRecord::Base
  include SecurePassword
  #...
end