Add additional attribute on a has_many through in Rails

Question

I have set up the following associations:

class BookLaunch < ApplicationRecord
  has_many :book_launch_locations, dependent: :destroy
  has_many :stores, through: :book_launch_locations
    
....

class Store < ApplicationRecord

  has_many :book_launch_locations
  has_many :book_launch, through: :book_launch_locations

....

class BookLaunchLocation < ApplicationRecord
  belongs_to :book_launch, :touch => true
  belongs_to :store
end

The BookLaunchLocation has an attribute called book_price.

  create_table "book_launch_locations", id: :integer, options: "ENGINE=InnoDB DEFAULT CHARSET=utf8", force: :cascade do |t|
    t.integer "book_launch_id", null: false
    t.integer "store_id", null: false
    t.integer "book_price", default: 1, null: false
    ...
    t.index ["book_launch_id", "store_id"], name: "pair_uniqueness", unique: true
    t.index ["book_launch_id"], name: "index_book_launch_locations_on_book_launch_id"
    t.index ["store_id"], name: "index_book_launch_locations_on_store_id"
  end 

I want to add book_price to stores in the BookLaunch model so that when I call @book_launch.stores, it will have all the attributes of stores + the book_price attribute.

Is this possible?

I'm calling it using fastJsonApi like this:

    options = {
      include: [:book_launch, :'book_launch.properties']}
    json = Api::launchSummarySerializer.new(summaryData, options).serialized_json

    render json: json

Answer 1

If you want to create a join model row with more then just two foreign key columns you need to create it explicitly instead of implicitly.

store = Store.first
book_launch = BookLaunch.first

BookLaunchLocation.create(
  store: store,
  book_launch: book_launch,
  price: 999
)

# or
store.book_launch_locations.create(
  book_launch: book_launch,
  price: 999
)

# or
book_launch.book_launch_locations.create(
  store: store,
  price: 999
)

Implicitly creating a join model row is when you create it through the indirect association:

Store.first.book_launches.create!(attributes)
BookLaunch.first.stores.create!(attributes)