ruby.rb

app/models/invoice_payment.rb
class InvoicePayment < ActiveRecord::Base
 def to_liquid
    InvoicePaymentDrop.new(self)
  end
end

app/drops/invoice_payment_drop.rb
class InvoicePaymentDrop < Liquid::Drop
  delegate :days_late, :duedate, :priceinbtw, :priceexbtw, :print_date, :id, :late, :unpayed, to: :invoice

  attr_reader :invoice

  def initialize(invoice)
    @invoice = invoice
  end
end

console: 
>> reload!;InvoicePayment.last.to_liquid
NameError: uninitialized constant InvoicePayment::InvoicePaymentDrop


####
class Invoice < ActiveRecord::Base
  def to_liquid
    InvoiceDrop.new(self)
  end
end

class InvoiceDrop < Liquid::Drop
  delegate :days_late, :duedate, :priceinbtw, :priceexbtw, :print_date, :id, :late, :unpayed, to: :invoice

  attr_reader :invoice

  def initialize(invoice)
    @invoice = invoice
  end
end
console:
>> reload!;Invoice.last.to_liquid
InvoiceDrop

with_errors.rb

class Invoice < ActiveRecord::Base
  def to_liquid
    InvoiceDrop.new(self)
  end
end

class InvoicePayment < ActiveRecord::Base
    def to_liquid
    InvoicePaymentDrop.new(self)
  end
end

Class InvoiceDropZ < Liquid::Drop #TYPO Z
  delegate :days_late, :duedate, :priceinbtw, :priceexbtw, :print_date, :id, :late, :unpayed, to: :invoice

  attr_reader :invoice

  def initialize(invoice)
    @invoice = invoice
  end
end

class InvoicePaymentDropZ < Liquid::Drop #TYPO Z
  delegate :id, :invoice_id, :amount, :description, :payment_type, :created_at, :updated_at, :user_id, to: :invoice_payment

  attr_reader :invoice_payment

  def initialize(invoice_payment)
    @invoice_payment = invoice_payment
  end
end


###

>> reload!;Invoice.last.to_liquid
Reloading...
  Invoice Load (0.2ms)  SELECT  `invoices`.* FROM `invoices`  ORDER BY `invoices`.`id` DESC LIMIT 1
LoadError: Unable to autoload constant InvoiceDrop, expected /home/thowou/rubyprojects/finance-dev/app/drops/invoice_drop.rb to define it

>> reload!;InvoicePayment.last.to_liquid
Reloading...
  InvoicePayment Load (0.2ms)  SELECT  `invoice_payments`.* FROM `invoice_payments`  ORDER BY `invoice_payments`.`id` DESC LIMIT 1
NameError: uninitialized constant InvoicePayment::InvoicePaymentDrop

with_errors2.rb

class InvoicePayment < ActiveRecord::Base #lowercase
  def to_liquid
    InvoicepaymentDrop.new(self)
  end
end

>> reload!;InvoicePayment.last.to_liquid
Reloading...
  InvoicePayment Load (0.2ms)  SELECT  `invoice_payments`.* FROM `invoice_payments`  ORDER BY `invoice_payments`.`id` DESC LIMIT 1
LoadError: Unable to autoload constant InvoicepaymentDrop, expected /home/thowou/rubyprojects/finance-dev/app/drops/invoicepayment_drop.rb to define it