Rails 3 grabbing the joined table rows. Can you do this?

I have a table called Delivery that looks like this:

class Delivery
...
has_many :delivery_products
has_many :products, through: delivery_products
end

DeliveryProducts is a joined table:

class DeliveryProduct
  belongs_to :delivery
  belongs_to :product

end

In my tests, I have some factories that create a delivery_product with a delivery:

create(:delivery_product, delivery: delivery, product: wine.product, product_slot: product_slot)

However, when I try to grab the delivery_products via the delivery, I can't seem to do this:

delivery.delivery_products 
=> []

However, delivery.products does indeed return me the items.

Is there an issue with grabbing the rows of the joined table?