Help with rails belongs_to

I'm stuck -

I'm building an accounting app in rails, and have a model where a customer will have multiple invoices as well as multiple payments (related to those invoices)

Here's a quick look at a simplified model:

class Customer < ActiveRecord::Base
  has_many :customer_payments
  has_many :invoices  
  accepts_nested_attributes_for :customer_payments, :allow_destroy => true
end

class CustomerPayment < ActiveRecord::Base
  has_many :customer_payment_开发者_高级运维items
  belongs_to :customer
  belongs_to :invoice
  accepts_nested_attributes_for :customer_payment_items
end

class CustomerPaymentItem < ActiveRecord::Base
  belongs_to :invoice, :inverse_of => :customer_payment_items
  belongs_to :customer_payment
end

 class Invoice < ActiveRecord::Base
  has_many :invoice_lines, :dependent => :destroy   
  has_many :customer_payment_items, :inverse_of => :invoice
  belongs_to :customer
  accepts_nested_attributes_for :invoice_lines, :allow_destroy => true
end

I have a nested form where I want to show Customer attributes, CustomerPayment attributes and CustomerPaymentItem attributes - which all works fine.

I also want to show Invoice attributes for each CustomerPaymentItem (each CustomerPaymentItem relates back to a single invoice) and while I can get a form to show the CustomerPaymentItem info, I can't get it to show Invoice information which is needed to give reference to the user. - I'm having trouble getting data from a belongs_to association to show on the form.

I'm at a loss - Shouldn't I be able to traverse the belongs_to association? FYI - I can send data to the log where I know the invoice data is populated during the CustomerPayment.new call, it seems to just get lost between the controller and the form.

How should I access that data? Here's the form info -- (coming from a couple of rendered forms) the stuff that doesn't show is in between the ---.

<p>
  <%= f.label :amount %><br />
  <%= f.text_field :amount %>
</p>
<p>
  <%= f.label :invoice_id %><br />
  <%= f.text_field :invoice_id %>
</p>

<p>
  <% f.fields_for :invoice do |builder| %>
    --- doesn't show
    Invoice Detail
    <p>
      <%= builder.label :number %><br />
      <%= builder.text_field :number %>
    </p>  
    <p>   
      <%= builder.label :sub_total %><br />
      <%= builder.text_field :sub_total %>
    </p>
    --- doesn't show

  <% end %>
</p>

Am I missing something in my fields_for to show the :invoice reference data? Is my model too complex for rails to make sense of?

@corroded was right - the issue was my lack of an = sign in

<% f.fields_for :invoice do |builder| %>

I guess everyone needs to learn that lesson