Question

Say am writing a function that will add a product to the shopping cart.

I have a cart.rb model, and the method signature looks like:

def self.add_product(store, user, product, quantity, ...)
  # store.id == product.store_id
  # quantity > 0 ?
  # product is active?
  # if product is in cart, update quantity

end

So I have to pass in around 4 other models, and then to some sanity checks also.

So if store.id != product.store_id, I want to return with some kind of an error or status saying the product doesn’t belong to this store so I can’t continue.

If the quanitity is 0, I want to tell the user the quantity has to be > 0.

etc.

Where should all this logic be? There are many other models involved so I’m very confused.

Should I use the vote error collection? Or pass back status codes?

What is the rails way? Please clarify.

Thanks!

Answer 1

To elaborate on my comment above, here’s how your Cart and CartItem classes might look/work.

class Cart < ActiveRecord::Base
  has_many    :items, :class_name => 'CartItem'
  belongs_to  :user   # one user per cart 
  belongs_to  :store  # and one store per cart 
end

class CartItem < ActiveRecord::Base
  belongs_to :cart
  belongs_to :product

  validates_presence_of :cart, :product

  # sanity check on quantity
  validates_numericality_of :quantity,  :greater_than => 0,
                                        :only_integer => true

  # custom validation, defined below
  validate :product_must_belong_to_store

  protected
  def product_must_belong_to_store
    unless product.store_id == cart.store_id
      errors.add "Invalid product for this store!"
    end
  end
end

# Usage:

# creating a new cart
cart = Cart.create :store => some_store, :user => some_user

# adding an item to the cart (since `cart` already knows the store and user
# we don't have to provide them here)
cart_item = cart.items.create :product => some_product, :quantity => 10