了解Rails活动记录关联

Question

I was reading a book for agile web development and I found this code

application controller

  private    
  def current_cart
    Cart.find(session[:cart_id])
  rescue ActiveRecord::RecordNotFound
    cart = Cart.create
    session[:cart_id] = cart.id
    cart
  end

line items controller

def create
  @cart = current_cart
  product = Product.find(params[:product_id])
  @line_item = @cart.line_items.build(:product => product)
  respond_to do |format|
   if @line_item.save
     format.html { redirect_to(@line_item.cart,
            :notice => 'Line item was successfully created.') }

I don't understand the following two lines

@line_item = @cart.line_items.build(:product => product)
format.html { redirect_to(@line_item.cart,
       :notice => 'Line item was successfully created.') }

@cart holds the value of cart_id then what @cart.line_items point and what is the use of build method here?

Also what @line_item.cart means here? which action will be called?

Answer 1

Okay, let's start with @cart . As phoet says, it is not a simple integer. It is set by Cart.find(session[:cart_id]) , so it is an instance of the Cart model based off a row in the database.

The Cart model presumably inherits from ActiveRecord::Base which has all the logic for has_many (and other) associations (relations) between models. So, I would expect to see at least

def Cart < ActiveRecord::Base
    has_many :line_items
end

in the Cart model.

So, when you call @cart.line_items you get something that is conceptually a list of that cart's line items through the association set up by the call to has_many . Calling build on that results in a new (as of yet, unpersisted) instance of the LineItem model that has its cart_id value set to @cart.id .

I hope that clears things up a bit. Rails associations are tricky business.

Answer 2

Your assumption is not correct. @cart holds a Cart object from the current_cart method. It's either new, or the one held in the session.

So @cart.line_items works with the has_many relation of Cart and line_items.build will setup a new LineItem to be saved to the database on @line_tem.save .