For example:

class Product
  has_many :sales_orders

  def total_items_deliverable
    self.sales_orders.each { |so| #sum the total }
    #give back the value
  end
end

class SalesOrder
  def self.deliverable
    # return array of sales_orders that are deliverable to customer
  end
end

1. SalesOrder.deliverable #give all sales_orders that are deliverable to customer
2. pa = Product.find(1)
3. pa.sales_orders.deliverable #give all sales_orders whose product_id is 1 and deliverable to customer
4. pa.total_so_deliverable

The very point that i'm going to ask is: how many times SalesOrder.deliverable is actually computed, from point 1, 3, and 4, They are computed 3 times that means 3 times access to database

so having total_so_deliverable is promoting a fat model, but more database access. Alternatively (in view) i could iterate while displaying the content, so i ends up only accessing the database 2 times instead of 3 times.

Any win win solution / best practice to this kind of problem ?