Question

I have some models and have views code that look like the following

class Address < ActiveRecord::Base
  belongs_to :customer
end

class Customer < ActiveRecord::Base
  has_one :address
  has_many :invoices
end

class Invoice < ActiveRecord::Base
  belongs_to :customer
end

This code shows a simple invoice structure, with a customer who has a single address.
The view code to display the address lines for the invoice would be as follows:

<%= @invoice.customer.name %>
<%= @invoice.customer.address.street %>
<%= @invoice.customer.address.city %>,
<%= @invoice.customer.address.state %>
<%= @invoice.customer.address.zip_code %>

above view code is not ideal. For proper encapsulation, the
invoice should not reach across the customer object to the street attribute of the
address object. Because if, for example, in the future your application were to change
so that a customer has both a billing address and a shipping address, every place in
your code that reached across these objects to retrieve the street would break and
would need to change. How can I avoid this problem ?

Answer 1

A simples solution would be the customer have a method that will return its main address, like:

class Customer < ActiveRecord::Base
  def main_address
    self.address
  end
end

If you access its address only by this method, when it has more than one address it is necessary just change the main_address method to do whatever you want to.

Edit 1:

Another option would be use the delegate as suggested by @soundar

class Invoice < ActiveRecord::Base
  belongs_to :customer
  delegate :address, :to => :customer
end