strange behavior database register rails 3 app

hi people

I have something strange here, and I would like to share it with you,
to find a solution or at least, different opinion.

I have a rails 3.0.9 app. I'm trying to store historical data so I
made the following.

I have 3 tables "invoices", "purchases", "detail_purchases". I'm using
"nested_attributes" (not nested form) in 2 levels deep (invoices
builds purchases and then purchases builds detail_purchases). Due to
this I have to use some callbacks, because I don't have controllers
for "purchases" nor "detail_purchases".

In the historical table I store

   - user_id (user who made the change)
   - object_id (in this case, the id of the invoice, id of purchase,
etc. depending on which one is changed or created)
   - class_name
   - action_name (create, update, destroy, etc.)
   - datetime

so, when I create an invoice, the logic order should be:

  1. create purchase
  2. create detail_purchase
  3. update purchase
  4. create invoice

but get the following, instead

   1. update purchase
   2. create detail_purchase
   3. create purchase
   4. create invoice

I check datetime, the order is correct, the registers are stored in
the database... but I still don't understand how can I "update"
something that is not yet created.

Hope you can help me