Question

I am writing a simple accounting system for managing costs. The structure is like this:

Invoice  - can have many products
  Product - can have many costs, also can act_as_tree 
      LineItem  - 
    Product
        LineItem
        LineItem
      Product
      LineItem
  Product
      LineItem
  LineItem

I had this set up as a has_many and belongs_to for the three classes but think that the following is more appropriate (based upon reading Rails 3 Way – any shortcomings are my lack of understanding; just trying to give context)

Class Invoice < ActiveRecord::Base
    has_many :products
    has_many :line_items, :through => :products
end

 Class Product < ActiveRecord::Base
    belongs_to :invoice
    belongs_to :line_item
 end

class LineItem < ActiveRecord::Base
    has_many :products
    has_many :invoices, :through => :invoices
end

But I don’t this is working correctly.

if I do the following:

>@i=Invoice.find(1)
>@i.products # this works
>@i.line_items # doesn't work, unidentified method line_items

This is the first time I’m using has_many :through. Is this set up correctly for my data model? Also, is it possible to use it in conjunction with acts_as_tree – I’d like to be able to say:

>@i.line_items 

and get back all the line items for that specific invoice. Possible?

thx for help

Answer 1

First a question: What is your relation between Product and LineItem: Has 1 product many line items or is 1 and the same line item referenced in many products? The rest of this answer is based on the assumption that every product should have multiple line items.

I think your models should be defined like that:

# No change
Class Invoice < ActiveRecord::Base
  has_many :products
  has_many :line_items, :through => :products
end

# Changed the relation to :line_items
Class Product < ActiveRecord::Base
  belongs_to :invoice
  has_many :line_items
end

class LineItem < ActiveRecord::Base
  belongs_to :products
  # The following then does not make sense
  ##has_many :invoices, :through => :invoices
end