How to find a object and insert it in a database - ruby-on-rails

Question

I’m trying to pick up the shop_id and subscription_id from a cart and insert/save them into the purchase database.
I have a cart, line_items and subscriptions.
And this is how I’m trying to do this:
#purchase = #cart.line_items.build
#purchase = current_shop.purchases.build(purchase_params)
#purchase.save!
I got the shop_id, but the subscription_id for some reason is nil.
Any ideas what maybe wrong?
Update 1
def purchase_params
params.permit(:subscription_id, :shop_id, :created_at, :updated_at, :id, :cart_id)
end
Started POST "/line_items?subscription_id=1" for ::1 at 2017-06-18 16:45:12 +0300
Processing by LineItemsController#create as HTML
Parameters: {"authenticity_token"=>"dUonc4AnCvFTuK1b+TAKho/kmpvl7XaOM7SGcNalzdQV1+CqhY4 p7znDiL/TV12pVKeDTqlR7j5NL65X1S/75A==", "subscription_id"=>"1"}
Cart Load (0.1ms) SELECT "carts".* FROM "carts" WHERE "carts"."id" = ? LIMIT ? [["id", 14], ["LIMIT", 1]]
(0.1ms) begin transaction
SQL (0.4ms) INSERT INTO "carts" ("created_at", "updated_at") VALUES (?, ?) [["created_at", 2017-06-18 13:45:12 UTC], ["updated_at", 2017-06-18 13:45:12 UTC]]
(0.7ms) commit transaction
Subscription Load (0.2ms) SELECT "subscriptions".* FROM "subscriptions" WHERE "subscriptions"."id" = ? LIMIT ? [["id", 1], ["LIMIT", 1]]
(0.1ms) begin transaction
SQL (0.6ms) INSERT INTO "line_items" ("subscription_id", "cart_id", "created_at", "updated_at") VALUES (?, ?, ?, ?) [["subscription_id", 1], ["cart_id", 18], ["created_at", 2017-06-18 13:45:12 UTC], ["updated_at", 2017-06-18 13:45:12 UTC]]
(0.9ms) commit transaction
Redirected to http://localhost:3000/carts/18
Completed 302 Found in 21ms (ActiveRecord: 3.2ms)
I'm implementing braintree payments via the following create method and this is where I have the small peace of code:
def create
current_shop.has_payment_info?
#result = Braintree::Transaction.sale(
amount: #cart.total_price,
payment_method_nonce: params[:payment_method_nonce],
customer: {
first_name: current_shop.first_name,
last_name: current_shop.last_name,
company: current_shop.shop_name,
email: current_shop.email,
phone: current_shop.phone_number,
website: current_shop.web_page
},
options: { store_in_vault: true })
if #result.success?
current_shop.update(braintree_customer_id: #result.transaction.customer_details.id) unless current_shop.has_payment_info?
#purchase = #cart.line_items.build
#purchase.save!
#purchase = current_shop.purchases.build(purchase_params)
#purchase.save!
#cart.destroy
redirect_to front_index_path, notice: 'Your transaction was succesfully processed'
else
gon.client_token = generate_client_token
redirect_to :back, :notice => 'Something went wrong while processing your transaction. Please try again!'
end
end
Update 2
class Purchase < ApplicationRecord
belongs_to :shop
belongs_to :cart
end
class Shop < ApplicationRecord
has_many :items
has_many :purchases
has_many :subscriptions
end
class Subscription < ApplicationRecord
has_many :line_items
belongs_to :shop
end
class Cart < ApplicationRecord
has_many :line_items, dependent: :destroy
has_many :purchases
end
class LineItem < ApplicationRecord
belongs_to :subscription
belongs_to :cart
end

Answer 1

Build here only links up a new line_item with #cart, like this, LineItem.new(cart_id: #cart.id). subscription_id is not linked here as #cart doesn't have a relationship with subscriptions.
#cart.line_items.build
line_item = #cart.line_items.build
fetch the desired subscription_id and assign it to 'line_item.subscription_id '
line_item.subscription_id = fetch_the_subscription_id
line_item.save!