Question

Taking this snippet from the famous agile web development with rails book (most recent edition):

def create 

  @cart = current_cart 

  product = Product.find(params[:product_id])

  @line_item = @cart.line_items.build(product: product)

This is for a general ecommerce/depot app, and this function is for the “Add to cart” button for a specific product. Here’s where I’m confused:

I imagined the code being:

@line_item = line_items.build(product: product) 

@line_item = line_items.build(cart: @cart) 

Basically If a line_item belongs to both a product and a cart, I create those relationships separately.

Also, if the cart doesn’t have any line_items yet, then how can I do @cart.line_items? I understand line_items.build(product: product) will return to me a line_item object (which I save into @line_item), but how does cart.line_items work if there’s no line_items in the cart yet?

Answer 1

In the Cart model, there is most likely a has_many association with the LineItem model called line_items. Rails will automatically initialize that association with an empty array.

You can think of @cart being initialized as:

@cart = Cart.new
@cart.line_items = []

Every time you add call @cart.line_items.build, think of it as this:

new_line_item = LineItem.new cart: @cart
@cart.line_items.push new_line_item

Note: This is absolutely not how the actual code works, but you can think of the functional behaviour as the same.