CODEWITHSUNDEEP
ruby-on-rails
Radhika
Radhika

Reputation: 2561

Using 'has_many' relation in rails

I am reading Agile Web Development with Rails. They have used the syntax:

item = cart.line_items.build(product_id: product_id)

My questions are:

  1. What exactly is happening in the above statement? Which parts of the program are called? I couldnt find the detailed explanation.

  2. why use build and then save instead of simply create?

Upvotes: 0

Views: 73

Answers (3)

fguillen
fguillen

Reputation: 38772

Check the documentation to see how many auto-generated methods are injected into the Object when you define a has_many relationship:

Among all of them it is the collection.build.

1) This method is just a helper so you only need to write one line for something that otherwise would need more than one.

2) Not special reason, depends on the context, sometimes you only want to initialize the object and not save it yet. If you want to save the object just after initialization use the other auto-generated method collection.create!

Upvotes: 1

steakchaser
steakchaser

Reputation: 5239

The above line creates a new line_item associated with the cart and is initialized with a product_id. Build is essentially a nice bit of DSL that lets you leverage already existing relationships for creating new objects. It is just utilizing the has_many: :line_items relation in the cart and the standard Object.new

The advantage to this approach is that you get the association with the cart for free instead of doing:

line_item = LineIten.new
line_item.cart = cart
line_item.product_id = product_id

Upvotes: 2

OneChillDude
OneChillDude

Reputation: 8006

The has_many :association_name method, when called in a model definition, creates several methods, one of which is the .association_name method, which makes a SQL query selecting all objects in that association.

To learn more about this, go to your terminal, type rails c, and you can start playing around with how this works.

Upvotes: 1

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