9 lines of code to add a :dependent => :restrict option to has_many (equivalent of "ON DELETE restrict")

has_many :dependent => :restrict

when :restrict
  method_name = "has_many_dependent_restrict_for_#{reflection.name}".to_sym
  define_method(method_name) do
    unless send(reflection.name).empty?
       # change to named error...
      raise ActiveRecord::StatementInvalid, "can't delete record because of dependent #{reflection.name}" 
    end
  end
  before_destroy method_name