Question

I’m not sure whether this is a problem of scopes or something, but I’m having problems with updating an object in a test:unit.

The interesting thing is that all functions work perfectly in development mode, but as soon as I switch to rake test:unit, it all breaks.

Here’s the relevant code:

  class Cart < ActiveRecord::Base

  def add_product(product)
    current_item = line_items.find_by_product_id(product.id)
    if current_item
      Rails::logger.debug "Incrementing quantity"
      Rails::logger.debug current_item.quantity
      current_item.quantity += 1
      Rails::logger.debug current_item.quantity
    else
      current_item = line_items.build(product_id: product.id,
                                     price: product.price)
    end
    Rails::logger.debug "Added Product"
    Rails::logger.debug current_item.quantity
    current_item
  end

and the associated test

  test "create new cart with one item added twice" do
    cart = Cart.new
    cart.add_product(products(:one)).save!
    assert_equal 1, cart.line_items.size
    assert_equal 36.00, cart.total_price
    Rails::logger.debug cart.line_items.to_a
    cart.add_product(products(:one)).save!
    Rails::logger.debug "Added second item"
    Rails::logger.debug cart.line_items.to_a
    Rails::logger.debug cart.total_price
    assert_equal 1, cart.line_items.size
    assert_equal 72.00, cart.total_price
  end

And here’s the log output:

Incrementing quantity
1
2
Added Product
2
   (0.1ms)  SAVEPOINT active_record_1
   (0.3ms)  UPDATE "line_items" SET "quantity" = 2, "updated_at" = '2013-01-18 15:27:06.958210' WHERE "line_items"."id" = 980190963
   (0.1ms)  RELEASE SAVEPOINT active_record_1
Added second item
[#<LineItem id: 980190963, product_id: 1, cart_id: nil, created_at: "2013-01-18 15:27:06", updated_at: "2013-01-18 15:27:06", quantity: 1, price: 36>]
36

ew quantity (which has changed according to the last log).
Interestingly, within the function itself, the quantity field is updating to two. It even saves to the SQL database. But when I call up cart.line_items, it even has the updated field, but it doesn’t save the new quantity (which has changed according to the last log).

Answer 1

Rails does not have an identity map by default. This means that whenever you do a LineItem.find(1) and another LineItem.find(1) you’ll get two separate objects that are created from the same row of data in your database but do not have any connection between them. If one of them is changed and saved to the database the other object won’t know about it and still have the old data.

In your example you’re doing line_items.find_by_product_id(product.id) which will do a find and return a new object every time. There’s no connection to any LineItem objects previously loaded into cart.line_items.

There are two options to update the data on stale objects. One of them is the .reload method which will reload all the data for one object from the database. The other option is passing a true to the cart.line_items association. A call like cart.line_items(true) will force a new query to the database to get all line items.

The reason that this bug only hits you during test is simple: In the development mode, inserting and reading is usually done in two separate requests and therefore all the objects are freshly loaded from the database every time.