Representing a has_many relationship in Rails with an abstract class - ruby-on-rails

Question

I have a relationship which I am having a hard time modelling.
I have a Subscription class which is a regular ActiveRecord model and it can have one or more PaymentSources. The problem however is that a payment source could refer to either a CreditCard or a BankAccount.
Given that these models have very different data associated with them I don't feel as though STI is a good option here. So I was wondering if there is an established or recommended approach for a situation in Rails where a model has_many of another model which is actually an abstraction for 2 or more classes which don't share the same data layout.
Ideally, in this particular example I could say something like subscription.payment_source.default and have it refer to either a CreditCard or a BankAccount depending on what the user had selected as their preferred billing method.

Answer 1

TLDR:
[Updated] After some pondering, I will do Option 2 (the more complete solution) which is future-proof flexible, but if you don't need all of this complexity, I'll do just Option 1.
Option 1:
class Subscription < ApplicationRecord
belongs_to :credit_card
belongs_to :bank_account
def payment_sources
[credit_card, bank_account].compact
end
def default_payment_source
case user.preferred_billing_method # assuming you have an integer column in users table called `preferred_billing_method`
when 0 then credit_card # asssuming 0 means "Credit Card"
when 1 then bank_account # assuming 1 means "Bank Account"
else NotImplementedError
end
end
end
Usage
Subscription.first.default_payment_source
# => returns either `CreditCard` or `BankAccount`, or `nil`
Subscription.first.payment_sources.first
# => returns either `CreditCard` or `BankAccount`, or `nil`
Option 2:
class User < ApplicationRecord
belongs_to :default_payment_source, class_name: 'PaymentSource'
has_many :subscriptions
end
class Subscription < ApplicationRecord
belongs_to :user
has_many :payment_sources_subscriptions
has_many :payment_sources, through: :payment_sources_subscriptions
end
# This is just a join-model
class PaymentSourcesSubscription < ApplicationRecord
belongs_to :subscription
belongs_to :payment_source
validates :subscription, uniqueness: { scope: :payment_source }
end
# this is your "abstract" model for "payment sources"
class PaymentSource < ApplicationRecord
belongs_to :payment_sourceable, polymorphic: true
has_many :payment_sources_subscriptions
has_many :subscriptions, through: :payment_sources_subscriptions
validates :payment_sourceable, uniqueness: true
end
class CreditCard < ApplicationRecord
has_one :payment_source, as: :payment_sourceable
end
class BankAccount < ApplicationRecord
has_one :payment_source, as: :payment_sourceable
end
Usage:
User.first.default_payment_source.payment_sourceable
# => returns either `CreditCard` or `BankAccount`, or `nil`
Subscription.first.payment_sources.first.payment_sourceable
# => returns either `CreditCard` or `BankAccount`, or `nil`