Scope vs Class Method in Rails 3

Question

Based on the Rails 3 API, the difference between a scope and a class method is almost non-existent.

class Shipment < ActiveRecord::Base
  def self.unshipped
    where(:shipped => false)
  end
end

is the same as

scope :unshipped, where(:shipped => false)

However, I'm finding that I'm sometimes getting different results using them.

While they both generate the same, correct SQL query, the scope doesn't always seem to return the correct values when called. It looks like this problem only occurs when its called the same way twice, albeit on a different shipment, in the method. The second time it's called, when using scope it returns the same thing it did the first time. Whereas if I use the class method it works correctly.

Is there some sort of query caching that occurs when using scope?

Edit:

order.line_items.unshipped

The line above is how the scope is being called. Orders have many line_items.

The generate_multiple_shipments method is being called twice because the test creates an order and generates the shipments to see how many there are. It then makes a change to the order and regenerates the shipments. However, group_by_ship_date returns the same results it did from the first iteration of the order.

def generate_multiple_shipments(order)
  line_items_by_date = group_by_ship_date(order.line_items.unshipped)

  line_items_by_date.keys.sort.map do |date|
    shipment = clone_from_order(order)
    shipment.ship_date = date
    line_items_by_date[date].each { |line_item| shipment.line_items << line_item }
    shipment
  end
end

def group_by_ship_date(line_items)    
  hash = {}
  line_items.each do |line_item|
    hash[line_item.ship_date] ||= []
    hash[line_item.ship_date] << line_item
  end
  hash
end

Can you please provide an example (snippet of code) where you believe the query gets cached when called twice. — George Yacoub, Jan 27 '12 at 03:19
Inspecting the Rails log will show you whether the result is being cached. — Sasha, Jan 30 '12 at 03:30

score 1 · Answer 1 · answered Jan 30 '12 at 03:36

1

Assuming that you are referencing Rails 3.1, a scope can be affected by the default scope that may be defined on your model whereas a class method will not be.

answered Jan 30 '12 at 03:36

Sasha

544
7
5

score 1 · Accepted Answer · answered Jan 30 '12 at 04:09

1

I think your invocation is incorrect. You should add so-called query method to execute the scope, such as all, first, last, i.e.:

order.line_items.unshipped.all

I've observed some inconsistencies, especially in rspec, that are avoided by adding the query method.

You didn't post your test code, so it's hard to say precisely, but my exeprience has been that after you modify associated records, you have to force a reload, as the query cache isn't always smart enough to detect a change. By passing true to the association, you can force the association to reload and the query to re-run:

order.line_items(true).unshipped.all

answered Jan 30 '12 at 04:09

Wolfram Arnold

7,159
5
44
64

Forcing the association to reload seems to have done the trick. I'm still not completely clear on how the query cache determines whether there has been a change but at least it's working now. Thanks! – blim8183 Jan 30 '12 at 20:06
@blim8183 I don't know the answer to this either, but I suspect that this may have been a design trade-off. Glad the reload worked for you. – Wolfram Arnold Jan 31 '12 at 01:18

Scope vs Class Method in Rails 3

2 Answers2