gistfile1.rb

module InvoiceFinders
  def paid
    klass.find_by_index(:business_paid, "#{owner.key}-true")
  end

  def unpaid
    klass.find_by_index(:business_paid, "#{owner.key}-false")
  end
end

class Business
  many :invoices, 
    using: :index, # [SDC] Not available yet, might co-opt "reference" type
    extend: InvoiceFinders
end

class Invoice
  property :ref, String
  property :total, BigDecimal
  property :paid_total, BigDecimal
  property :business_key, String

  one :business, using: :stored_key

  index :business_ref, String do
    [business_key, ref].join '-'
  end  
  
  index :business_paid, String do
    "#{business_key}-#{paid_total >= total}"
  end
end


b = business.find(1)
# [SDC] This might work if the yet-to-be-implemented index relationship type does it properly
b.invoices.count # 3
# [SDC] These are currently inefficient because they will load all related documents to do the count
b.invoices.paid.count #2
b.invoices.unpaid.count #1