ruby on rails - :counter_cache for total items -

I have created a simple set of two related tables of 'order' in which many 'line_times' are related to a line item The quantity is also, like

order 1
line_a a: 'knitting basket for beginners', quantity: 3
line_itum b: 'a dummy guide for vampirism' quantity: 1

When I install the migration, then I can use the amount in it:

  Order.find (: all) .each do | O | O.update_attribute: line_items_count, o.line_items.map (& amp; quantity) .sum end   
 which gives me the correct number (4), but I do this on this order model Because I'm unable to exceed line item volume, and so this line calculates the number of items (2).  
 Therefore, line_item is in model I:  
  related_ to: command,: counter_cache = & gt; Is true   
 Is there any way I can specify the quantity so that it's right to say 4 instead of 2?  
 
 The feature 'counter_cache` is for maintaining counting of dependent objects (not yoga).  
 You can easily get it by writing a few lines of Ruby Code.  
 Assume that your  order  has the  line_items_sum  column in the table. The value of this column should be a default of 0.  
  class AddLineItemsSumToOrder & lt; ActiveRecord :: Migration DF self.up add_column: command,: line_items_sum,: integer,: default => 0 end DEIF self.down removal_column: command, line_items_sum end-end class command & lt; ActiveRecord :: Base is_my: line_time end   
 Now add callback to the  LineItem  class.  
  class LineItem