Question about a model example

I’m a beginner to both Ruby and Rails and I’m currently reading the Agile development with Rails, in which I’m currently developing the cart of the store.

I have a line_items model which belongs_to :products. This makes sense. Later in the example we use this code to check if a product is referenced by any line items before we destroy it:

class Product < ActiveRecord::Base
has_many :line_items

before_destroy :ensure_not_referenced_by_any_line_item


# ensure that there are no line items referencing this product
def ensure_not_referenced_by_any_line_item
if line_items.empty?
return true
errors.add(:base, 'Line Items present')
return false

This makes sense to me except of one part: if line_items.empty? I can only guess that *line_items* returns all the rows of the "line_items" table that contain the of the currently instantiated Product object, is that right? But how does the model knows what to fetch just by "line_items"? Isn't that too little info that we give to our model, regarding the logic of the task it has to do? Don't we have to declare somewhere something like: *return false if ==* ?

Thanks in advance people!

The fact that you have said has_many :line_items automatically makes a
method line_items available for any product that returns an array
(actually it is not strictly an array, but near enough) containing all
the line items for that product.

Similarly if you have a line item in @line_item then you can say
@line_item.product to get the associated product.

Have a look at the Rails Guide on ActiveRecord associations to find
all the methods that rails makes available.


I see.

Now I’ve upgraded my seeds.rb file, to add some entry into the line_items table and see how it works.

Actually I’ve added this line: LineItem.create (product_id: ‘8’, cart_id: ‘1’)

but when I run rake db:seed I get this error:

LineItem.create (product_id: 8, cart_id: 1)
rake db:seed
rake aborted!
/rails/depot/db/seeds.rb:55: syntax error, unexpected tLABEL

My migration file for LineItems table is this:

Remove the space before the ‘(’

LineItem.create(product_id: 8, cart_id: 1)

In newer versions of Ruby (1.9.3 and maybe earlier), this is not longer allowed.


Thank you! Silly me for missing that.