nullify_blank.rb

# config/initializers/nullify_blank.rb

class ActiveRecord::Base
  def self.nullify_blank(*args)
    for arg in args
      define_method :"#{arg.to_s}=" do |v|
        v = nil if v.blank?
        self[arg.to_sym] = v
      end
    end
  end
end

# model
class User < ActiveRecord::Base
  nullify_blank :author, :content
end