inverse_of_example.rb

# === Sem o inverse_of
c = Customer.first
o = c.orders.first
c.first_name == o.customer.first_name # => true
c.first_name = 'Manny'
c.first_name == o.customer.first_name # => false

# === Com o inverse_of
class Customer < ActiveRecord::Base
  has_many :orders, inverse_of: :customer
end
 
class Order < ActiveRecord::Base
  belongs_to :customer, inverse_of: :orders
end

c = Customer.first
o = c.orders.first
c.first_name == o.customer.first_name # => true
c.first_name = 'Manny'
c.first_name == o.customer.first_name # => true

# Fonte: http://guides.rubyonrails.org/association_basics.html#bi-directional-associations