Question

I have a rails model where I want to update an attribute by calculating the current value of that attribute plus the value I’m sending as a parameter. The following will update the record and overwrite the existing value for the attribute

def self.mark_receipt(qty_received,client_id,product_code)
  Product.where(:client_id => client_id, :product_code => product_code)
         .update_all(:qty_on_hand => :qty_received,
                     :qty_received => qty_received)
end

I want something more like this, but the following syntax doesn’t work.

def self.mark_receipt(qty_received,client_id,product_code)
  Product.where(:client_id => client_id, :product_code => product_code)
         .update_all(:qty_on_hand => :qty_on_hand + qty_received,
                     :qty_received => :qty_received + qty_received)
end

EDIT

Getting really close with the below answer, but this is what sql is running in the background

Product Load (0.5ms)  SELECT `products`.* FROM `products` 
                      WHERE `products`.`client_id` = 1 AND `products`.`product_code` = '88-89c'
  SQL (0.6ms)         UPDATE `products` 
                      SET `qty_on_hand` = 165, `qty_received` = 165

Instead of

 UPDATE `products`
 SET `qty_on_hand` = 165, `qty_received` = 165
 WHERE `products`.`client_id` = 1 AND `products`.`product_code` = '88-89c'

Answer 1

I believe this would work based on docs
http://apidock.com/rails/ActiveRecord/Relation/update_all

Product.update_all(['qty_on_hand = qty_on_hand + ? , qty_received = qty_received + ?', qty_received, qty_received],
                    {:client_id => client_id, :product_code => product_code)})

Notice that this would make the add operation at the database.