问使用抽象类在Rails中表示has_many关系

EN

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

Subscription.first.default_payment_source
# => returns either `CreditCard` or `BankAccount`, or `nil`

Subscription.first.payment_sources.first
# => returns either `CreditCard` or `BankAccount`, or `nil`

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

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`