With a nested routes like that,

get_barcode_ticket_orders POST   /tickets/:ticket_id/orders/get_barcode(.:format) orders#get_barcode

You would need to also pass the parameter ticket_id into the data hash.  Is the ticket_id available at this point? If so, you could pass it into a data attribute and access it in the JS. Otherwise, you may want to reconsider the routes to look something like this (or similar):

resources :tickets do
  resources :orders
end
post '/orders/get_barcode',  as: :get_barcode, controller: :orders, action: :get_barcode
# get_barcode POST   /orders/get_barcode(.:format)         orders#get_barcode

Since you don't have the ticket_id, it's not a valid route.

In your HTML, if the ticket_id is available and you're already displaying it somewhere then you can create a data attribute like

<%= tag :div, id: 'some_name', data: { 'ticket-id': @ticket.id } %>

You can access the value of the ticket's id in jQuery like

var ticket_id = $('#some_name').data('ticket-id');

And then in the AJAX POST

data: { ticket_id: ticket_id, upc: code }

Looks like you should remove the collection on the orders and instead do something like this

post '/orders/get_barcode',  as: :get_barcode, controller: :orders, action: :get_barcode

Since it sounds like you have a barcode which references a record in the Order model, you're not really going to be able to do a nested resource and have the collection on the Orders. Instead, you can create a manual path (as shown above) which goes to your orders#get_barcode action.

In the get_barcode action, you can look up the order and then get the ticket association.

It might look something like

def get_barcode
  @ticket = Order.includes(:ticket).find_by(barcode: params[:upc]).ticket
end

This is assuming that a Ticket has_many Orders and an Order belongs_to a Ticket.

The previous example will generate two queries and could be a bit slower. You could get fancy with your query and also do something like this which will create an inner join

@ticket = Ticket.joins(:orders).where(orders: {barcode: params[:upc]})

I'd recommend using https://aws.amazon.com/elasticsearch-service if hosting your instances in AWS. Just be mindful and lock the security of the search instance/cluster to the EC2 instance.

You could add https://github.com/rails/rails/blob/master/activerecord/lib/active_record/enum.rb to your lib folder and have it loaded in your path on the app boot. It should give you the functionality of enum prefix. I did something similar like this before I had fully upgraded a few Rails 4 apps to Rails 5.