Update attribute in callback Rails 3

Question

Following on from this question, I have spent the day trying to add a cumulative running sales total to my sales table. It's a bit tricky (for me) because I want a running total for sales where the isbn_id is the same, and, within that set, records where the channel_id is the same - ranked by invoice_date. This is all so I can calculate royalties on a particular range of units sold.

Here's my non-working callback code, in the Sale model:

before_save :runningtotal

private

def runningtotal
  @sale = Sale.order("invoice_date ASC")
  @lastbal = @sale.find_all_by_isbn_id(@isbn).group_by(&:channel)
  #that sucessfully gets all sales ranked by date ascending, then groups them by channel, just for the current isbn.
  @lastbal.each do |channel, sale|
    sale.each_with_index do |sale, i|
      previous_sale = sale[i-1] unless i==0
      next unless previous_sale
      @total_quantity = previous_sale.quantity + :quantity
      write_attribute(:total_quantity,@total_quantity)
    end
  end 
end  

Is this roughly how a callback should be written - just in the model? Does it just run magically before_save of a new sale?

My core question is: how can I update the attribute "total_quantity" to be the sum of "quantity" for the current record, and "total_quantity" for the previous record by date, in a before_save callback, within the constraints of the finds for isbn_id and channel_id?

Here's the output of the find:

ruby-1.9.2-p180 :025 > @lastbal = @sale.find_all_by_isbn_id(@isbn).group_by(&:channel) 
=> {#<Channel id: 4, isbn_id: nil, channel_name: "Gratis", created_at: "2011-05-26 11:08:22", updated_at: "2011-05-26 11:08:22">=>[#<Sale id: 26, isbn_id: 2, quantity: 10000, value: 40000, currency: "", total_quantity: nil, created_at: "2011-05-26 11:11:30", updated_at: "2011-05-26 11:11:30", customer: "6", retail_price: nil, discount: nil, channel_id: 4, invoice_date: "2011-05-18", rule_id: nil, trenche: nil>], #<Channel id: 1, isbn_id: nil, channel_name: "Home", created_at: "2011-05-16 19:47:27", updated_at: "2011-05-16 19:47:27">=>[#<Sale id: 22, isbn_id: 2, quantity: 1000, value: 5193, currency: "", total_quantity: nil, created_at: "2011-05-25 19:46:03", updated_at: "2011-05-25 19:46:03", customer: "a", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2011-05-11", rule_id: nil, trenche: nil>, #<Sale id: 24, isbn_id: 2, quantity: 1000, value: 4394, currency: "", total_quantity: nil, created_at: "2011-05-26 09:48:16", updated_at: "2011-05-26 09:48:16", customer: "g", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2011-05-10", rule_id: nil, trenche: nil>, #<Sale id: 25, isbn_id: 2, quantity: 1000, value: 4394, currency: "", total_quantity: nil, created_at: "2011-05-26 10:02:38", updated_at: "2011-05-26 10:02:38", customer: "g", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2011-05-05", rule_id: nil, trenche: nil>, #<Sale id: 21, isbn_id: 2, quantity: 1000, value: 5193, currency: "", total_quantity: nil, created_at: "2011-05-25 14:12:45", updated_at: "2011-05-25 14:12:45", customer: "a", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2010-10-15", rule_id: nil, trenche: nil>, #<Sale id: 13, isbn_id: 2, quantity: 50, value: 159, currency: "", total_quantity: nil, created_at: "2011-05-25 12:33:09", updated_at: "2011-05-25 12:33:09", customer: "a", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2010-01-01", rule_id: nil, trenche: nil>, #<Sale id: 14, isbn_id: 2, quantity: 25, value: 129, currency: "", total_quantity: nil, created_at: "2011-05-25 12:33:23", updated_at: "2011-05-25 12:33:23", customer: "", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2010-01-01", rule_id: nil, trenche: nil>, #<Sale id: 12, isbn_id: 2, quantity: 100, value: 415, currency: "", total_quantity: nil, created_at: "2011-05-25 12:32:50", updated_at: "2011-05-25 15:13:21", customer: "a", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2001-10-01", rule_id: nil, trenche: nil>, #<Sale id: 11, isbn_id: 2, quantity: 500, value: 2197, currency: "", total_quantity: nil, created_at: "2011-05-25 12:32:24", updated_at: "2011-05-25 15:11:20", customer: "a", retail_price: nil, discount: nil, channel_id: 1, invoice_date: "2000-10-01", rule_id: nil, trenche: nil>], #<Channel id: 2, isbn_id: nil, channel_name: "Export", created_at: "2011-05-16 19:47:35", updated_at: "2011-05-16 19:47:35">=>[#<Sale id: 23, isbn_id: 2, quantity: 2000, value: 5000, currency: "", total_quantity: nil, created_at: "2011-05-26 09:16:15", updated_at: "2011-05-26 09:16:15", customer: "v", retail_price: nil, discount: nil, channel_id: 2, invoice_date: "2011-05-02", rule_id: nil, trenche: nil>, #<Sale id: 17, isbn_id: 2, quantity: 242, value: 657, currency: "", total_quantity: nil, created_at: "2011-05-25 12:34:24", updated_at: "2011-05-25 12:34:24", customer: "b ", retail_price: nil, discount: nil, channel_id: 2, invoice_date: "2010-10-15", rule_id: nil, trenche: nil>, #<Sale id: 18, isbn_id: 2, quantity: 54, value: 194, currency: "", total_quantity: nil, created_at: "2011-05-25 12:34:44", updated_at: "2011-05-25 12:34:44", customer: "b ", retail_price: nil, discount: nil, channel_id: 2, invoice_date: "2010-10-15", rule_id: nil, trenche: nil>, #<Sale id: 15, isbn_id: 2, quantity: 135, value: 377, currency: "", total_quantity: nil, created_at: "2011-05-25 12:33:48", updated_at: "2011-05-25 12:33:48", customer: "b ", retail_price: nil, discount: nil, channel_id: 2, invoice_date: "2010-09-15", rule_id: nil, trenche: nil>, #<Sale id: 16, isbn_id: 2, quantity: 433, value: 830, currency: "", total_quantity: nil, created_at: "2011-05-25 12:34:06", updated_at: "2011-05-25 12:34:06", customer: "b ", retail_price: nil, discount: nil, channel_id: 2, invoice_date: "2010-09-15", rule_id: nil, trenche: nil>]} 

Here's the columns in my Sale model:

#  id             :integer         not null, primary key
#  isbn_id        :integer
#  quantity       :integer
#  value          :integer
#  currency       :string(255)
#  total_quantity :integer
#  created_at     :datetime
#  updated_at     :datetime
#  customer       :string(255)
#  retail_price   :integer
#  discount       :decimal(, )
#  channel_id     :integer
#  invoice_date   :date
#  rule_id        :integer

Thanks so much in advance.

UPDATE: final solution.

Really not sure that this counts as 'giving back to the community' as it's comically verbose, not DRY, full of puts which I used to figure out all the bugs, and badly formatted to boot, but heck, I'm a noob at and the very least I can come back here and laugh at myself in a few years when I know what I'm doing. So, here's my final solution, in Sale.rb. Poor overstuffed model. I will refactor this, one day.

before_save :runningtotal
after_commit :refresh

private
  def runningtotal
    # get the latest sale that matches the new sale's isbn and channel id, then rank by invoice date descending, and get the first record:
    lastsale = Sale.where(:isbn_id => self.isbn_id).where(:channel_id => self.channel_id).order("invoice_date DESC").first 
    allsales = Sale.where(:isbn_id => self.isbn_id).where(:channel_id => self.channel_id).order("invoice_date DESC")
    # set the total_quantity field in the new sales record to its quantity + the last sale's total.             
        if allsales.maximum(:invoice_date).nil? 
          puts "runningtotal thinks the max of invoice date in the allsales relation is nil"
          puts "and runningtotal is setting total_quantity on the new sale to be #{self.quantity + (lastsale.try(:total_quantity) || 0)}"
          self.total_quantity = self.quantity + (lastsale.try(:total_quantity) || 0)
          else    
            if self.invoice_date < allsales.maximum(:invoice_date)
            puts "the runningtotal method has been skipped because runningtotal thinks the current invoice date is less than the highest invoice date in the allsales relation"
            else 
              puts "this is a normal entry so runningtotal has set the total quantity to be #{self.quantity + (lastsale.try(:total_quantity) || 0) }"
              self.total_quantity = self.quantity + (lastsale.try(:total_quantity) || 0)     
            end
        end
  end

  def refresh
     allsales = Sale.where(:isbn_id => self.isbn_id).where(:channel_id => self.channel_id).order("invoice_date ASC")
       #if the runningtotal callback hasn't run, the total quantity will be nil, and nil triggers this after_commit callback 
       if total_quantity.nil?
         puts "running refresh callback"
         puts "here's a sample parameter pass: id: #{id} quantity: #{quantity} date: #{invoice_date} "
         puts "allsales class is #{allsales.class}"
         # if the new sale that's being saved has a date that's before any previous sale... 
            puts "before the if, refresh thinks that the earliest invoice date is #{allsales.minimum(:invoice_date)} and that invoice date is #{invoice_date}"
            if invoice_date <= allsales.minimum(:invoice_date)
              puts "date earlier than existing sales dates"
              puts "refresh thinks that the earliest invoice date is #{allsales.minimum(:invoice_date)} and that invoice date is #{invoice_date}"
              #... then set its total_quantity to the sale quantity... 

                update_attribute(:total_quantity, quantity)
                puts "total_qty updated with qty"
                # ... and update all subsequent records' total_quantity (skipping the before_save callback which would trigger an infinite loop).
                  allsales.each_with_index do |sale, i|
                    previous_sale = allsales[i-1] unless i==0
                    next unless previous_sale 
                     puts "getting qty out of arel when date earlier than others: #{previous_sale.quantity}"
                     puts "this is adding #{quantity} to #{previous_sale.quantity } which is #{quantity + previous_sale.total_quantity }" 
                 Sale.skip_callback(:save, :before, :runningtotal )
                    sale.update_attribute(:total_quantity, (sale.quantity + previous_sale.total_quantity ))
                 Sale.set_callback(:save, :before, :runningtotal)
                 end
            else
              # if the invoice date is within the min and max range of the previous sales...                
                # ... update all previous and subsequent records' total_quantity (skipping the before_save callback which would trigger an infinite loop).
                  allsales.each_with_index do |sale, i|                 
                    previous_sale = allsales[i-1] unless i==0
                    next unless previous_sale 
                    puts "getting qty out of arel within existing date range: #{previous_sale.quantity}"
                    puts "this is adding #{quantity} to #{previous_sale.quantity } which is #{quantity + previous_sale.total_quantity }" 
                Sale.skip_callback(:save, :before, :runningtotal )
                    sale.update_attribute(:total_quantity, (sale.quantity + previous_sale.total_quantity )) 
                Sale.set_callback(:save, :before, :runningtotal )
                  end
        end 
      end 
  end

Answer 1

Yes, using before_save in the model will run that every time it is saved, whether new or updated. Thus you need to watch out in calculations the expect the current (new) record to not exist yet. ;) You might want to use before_save, :on => :create to limit it to the creation action.

However, if I understand your english statement of the problem, your code is rather convoluted. I don't even see where @isbn is set, that could be dangerous...

Does this need to update the total on other objects this isbn and channel? Usually it's better form to simply calculate that as needed rather than trying to cache the total in every record.

within the callback, self is the current (new?) record, so use it to refer to new values.

  @sale = Sale.order("invoice_date ASC")
  @lastbal = @sale.find_all_by_isbn_id(@isbn).group_by(&:channel)

can be replaced by this, I think:

 @lastbal = Sale.order("invoice_date ASC").where(:isbn_id => self.isbn_id).group_by(&:channel)

I'm assuming that @isbn is actually the new record's isbn.

From there, I'm not sure if you are only intending to update the new record or the old ones... If you want to update the current record, just set the attribute and exit the callback, and it will be saved when the rest is saved: self.total_quantity = previous_sale.quantity + self.quantity

If you are intending to update the other objects too, then we have to update those objects and save them. I don't see that happening at all here in your code.

Your code goes through several loops, possibly hitting the write_attribute several times... that doesn't make sense.

If you mean you want to find the last record that matches the current isbn and channel to update the new record, here's what I would do:

def runningtotal
  lastsale = Sale.where(:isbn_id => self.isbn_id).
                  where(:channel_id => self.channel_id).
                  order("invoice_date DESC").first 
             # that should be the latest sale that matches 
             # the current isbn and channel
  self.total_quantity = self.quantity + (lastsale.try(:total_quantity) || 0) 
      # watch out for nil if no previous record exists ^
end  

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