Question

I currently have two models, List and Item:

class List < ApplicationRecord
  has_and_belongs_to_many :items
end

class Item < ApplicationRecord
  has_and_belongs_to_many :lists
end

Each list has_and_belongs_to_many items and each item has_and_belongs_to_many lists through the items_lists join table.

Currently, I can add items to the list using the following code:

list.items.new(id: item.id)

This works fine, but I would like to specify a quantity of the item when it is added to the list.

So, in the join table, I added another column called quantity that should store a quantity that shows however many of the item is needed. After adding the quantity column, I tried the following code to save the quantity along with a item:

list.items.new(id: item.id, quantity: 3)

However, I received an error that said ActiveModel::UnknownAttributeError: unknown attribute 'quantity' for Item.

Since the way I tried appears to be incorrect, what should be done to allow for a quantity of items to be added to a list?

Answer 1

you need to use a has many through relationship instead of the has and belongs to many. This will involve changing the relations on your two existing models as well as creating a third. You want to use this relationship for the many-to-many because you are storing data in addition to the two record primary keys (has_many_through documentation)

class List < ApplicationRecord
  has_many :items_lists
  has_many :items, through: :items_lists
end

class Item < ApplicationRecord
  has_many :items_lists
  has_many :lists, through: :items_lists
end

class ItemsList < ApplicationRecord
  belongs_to :item
  belongs_to :list
end

additionally, your items_lists table should have a list_id and an item_id. so you would end up calling

list.items.new(item_id: item.id, quantity: 3)

Ruby on Rails Join Table with Quantity

1 个答案: