http://api.shopify.com/collect.html This is actually a has_many :through join table between Product and CustomCollection. I don't support has_many :through as such yet, so you just get naiive access to join model itself (use Where/query parameters on Product/Collection to follow the relation). https://trello.com/card/has-many-through-collects-product-custom-collection/50a1c9c990c4980e0600178b/39