CODEWITHSUNDEEP
ruby-on-railsrubyactiverecord
Nils
Nils

Reputation: 2215

Shopping cart Active Record Associations

I am trying to implement a shopping cart within a Rails application. For this, I am trying to glue together these models:

user.rb

class User < ApplicationRecord
  has_many :cart_contents
end

cart_content.rb:

class CartContent < ApplicationRecord
  belongs_to :user
  has_one :product
end

product.rb

class Product < ApplicationRecord
end

To test if this arrangement is working, I tried this in the console:

irb(main):006:0> cc = CartContent.new
=> 
#<CartContent:0x000055679d802a28
...

irb(main):008:0> cc.user = User.find(1)
  User Load (0.2ms)  SELECT "users".* FROM [...]
=> #<User id: 1, email: "[email protected]", ...

irb(main):010:0> cc.product = Product.find(1)
  Product Load (0.1ms)  SELECT "products".* FROM [...]                                                                      
/[...]/activemodel-7.0.0/lib/active_model/attribute.rb:211:
in `with_value_from_database': can't write unknown attribute `cart_content_id` (ActiveModel::MissingAttributeError)

What am I missing here? Do I need to indicate a relationship to cart_content in product.rb?

Upvotes: 1

Views: 354

Answers (3)

Nils
Nils

Reputation: 2215

I found a solution that works for me.

product.rb

class Product < ApplicationRecord
  has_one :cart_content
end

cart_content.rb

class CartContent < ApplicationRecord
  belongs_to :user
  belongs_to :product
end

user.rb

class User < ApplicationRecord
  has_many :cart_contents
  has_many :products, through: :cart_contents # for convenience
end

Rails no longer expects a cart_content_id column on Product and the following commands all work in the console:

user = User.first
product = Product.first
cc = CartContent.new
cc.user = user
cc.product = product
cc.save

User.first.products # returns products currently in cart

Upvotes: 0

Marelons
Marelons

Reputation: 160

For this, you need to build relations between your models. As mentioned by Jared in this link for the foreign key to exist, you need to specify a belongs_to constraint.

The distinction is in where you place the foreign key (it goes on the table for the class declaring the belongs_to association)

class Product < ApplicationRecord
  belongs_to :cart_content
end

If you also want to access all Products for User you can go for below, but you probably don't need a through relation.

class User < ApplicationRecord
  has_many :cart_contents
  has_many :products, through: :cart_contents
end

Then your Product model would look like this

class Product < ApplicationRecord
  belongs_to :cart_content
  belongs_to :user
end

Upvotes: 0

Jared Beck
Jared Beck

Reputation: 17538

One possible solution: you may want your CartContent to belongs_to :product.

class CartContent < ApplicationRecord
  belongs_to :user
  has_one :product # change this to belongs_to
end

cc.product = Product.find(1)

# .. can't write unknown attribute `cart_content_id` (ActiveModel::MissingAttributeError)

has_one will expect your products table to have a cart_content_id column.

See guide section 2.7 Choosing Between belongs_to and has_one

Upvotes: 2

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