http://robots.thoughtbot.com/post/27572137956/tell-dont-ask

tell-dont-ask.rb

# Not so good:
def street_name(user)
  if user.address
    user.address.street_name
  else
    'No street name on file'
  end
end

# Better:
def street_name(user)
  user.address.street_name
end

class User
  def address
    @address || NullAddress.new
  end
end

class NullAddress
  def street_name
    'No street name on file'
  end
end