association_method_override.rb

DB.create_table(:subscriptions) do
  primary_key :id
end

DB.create_table(:discounts) do
  primary_key :id
  column :valid_until, Date
  foreign_key :subscription_id, :subscriptions
end

class Subscription < Sequel::Model
  one_to_many :discounts

  one_to_one :active_discount, class: :Discount do |ds|
    ds.where { valid_until >= Sequel.delay { |ds| ds.opts[:invoice_date] || Sequel::CURRENT_TIMESTAMP }}
  end

  # Is there a better way than using an alias here? super won't work inside the method
  alias_method :original_active_discount, :active_discount

  def active_discount(invoice_date = nil)
    if invoice_date
      original_active_discount { |ds| ds.clone(invoice_date: invoice_date) }
    else
      original_active_discount
    end
  end
end

class Discount < Sequel::Model
end

subscription = Subscription.create
subscription.add_discount(valid_until: Date.today + 3)

pp Hash[
  DEFAULT_ACTIVE_DISCOUNT: Subscription.first.active_discount,
  RESTRICTED_ACTIVE_DISCOUNT: Subscription.first.active_discount(Date.today)
]