Rails 4 STI inheritance - ruby-on-rails

Question

I'm using single table inheritance successfully like so:
class Transaction < ActiveRecord::Base
belongs_to :order
end
class Purchase < Transaction
end
class Refund < Transaction
end
The abbreviated/simplified PurchaseController looks like this:
class PurchaseController < TransactionController
def new
#transaction = #order.purchases.new(type: type)
end
def create
#transaction = #order.purchases.new secure_params
if #transaction.save
redirect_to #order
else
render :new
end
end
end
The abbreviated/simplified Purchase model looks like this:
class Purchase < Transaction
attr_accessor :cc_number, :cc_expiry, :cc_csv
end
What I'm trying to do is have different variations of a purchase, for instance a cash purchase & a cheque purchase. The issue is I'm not sure how to call the model for that variation.
For example:
class Cash < Purchase
attr_accessor :receipt_no
end
class CashController < TransactionController
def new
# This will use the Purchase.rb model so that's no good because I need the Cash.rb model attributes
#transaction = #order.purchases.new(type: type)
# This gives me the following error:
# ActiveRecord::SubclassNotFound: Invalid single-table inheritance type: Purchase is not a subclass of Cash
#transaction = Cash.new(type: 'Purchase', order: #order.id)
end
end

Answer 1

I'm not sure why it doesn't work for you, this works fine for me:
#order.purchases.new(type: "Cash") # returns a new Cash instance
You can also push a new Cash on to the association if you are ready to save it:
#order.purchases << Cash.new
Or you can define a separate association in Order:
class Order < ActiveRecord::Base
has_many :cashes
end
#order.cashes.new # returns a new Cash instance

Answer 2

Class
Maybe I'm being obtuse, but perhaps you'll be willing to not make the Purchase type an inherited class?
The problem I see is that you're calling Cash.new, when really you may be better to include all the functionality you require in the Purchase model, which will then be able to be re-factored afterwards.
Specifically, why don't you just include your own type attribute in your Purchase model, which you'll then be able to use with the following setup:
#app/controllers/cash_controller.rb
class CashController < ApplicationController
def new
#transaction = Purchase.new
end
def create
#transaction = Purchase.new transaction_params
#transaction.type ||= "cash"
end
private
def cash_params
params.require(:transaction).permit(:x, :y, :z)
end
end
The only downside to this would be that if you wanted to include different business logic for each type of purchase, you'll still want to use your inherited model. However, you could simply split the functionality in the before_create callback:
#app/models/puchase.rb
class Purchase < Transaction
before_create :set_type
private
def set_type
if type =="cash"
# do something here
else
# do something here
end
end
end
As such, right now, I think your use of two separate models (Cash and Cheque) will likely be causing much more of an issue than is present. Although I'd love to see how you could inherit from an inherited Model, what I've provided is something you also may wish to look into