invoice_drop.rb

# app/drops/invoice_drop.rb

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

  attr_reader :invoice

  def initialize(invoice)
    @invoice = invoice
  end
end

liquid.html.liquid

DOES NOT WORK:

  {% for invoice in relation.invoices.late %}
    <tr>
      <td>{{invoice.id}}</td>
      <td>{{invoice.print_date}}</td>
      <td>{{invoice.days_late}}</td>
      <td>{{invoice.priceinbtw}}</td>
    </tr>
    {% endfor %}
    
    DOES WORK
      {% for invoice in relation.invoices %}
    <tr>
      <td>{{invoice.id}}</td>
      <td>{{invoice.print_date}}</td>
      <td>{{invoice.days_late}}</td>
      <td>{{invoice.priceinbtw}}</td>
    </tr>
    {% endfor %}

model.rb

class Relation < ActiveRecord::Base
  has_many :invoices
  
  def to_liquid
    RelationDrop.new(self)
  end
end

class Invoice < ActiveRecord::Base
  scope :late, -> {unpayed.where("print_date < ? ", (Time.now-APP_CONFIG['betaalinformatie']['termijn'].days))}
  liquid_methods :days_late, :priceinbtw, :print_date, :id, :late, :unpayed

  def to_liquid
    InvoiceDrop.new(self)
  end
end

relation_drop.rb

# app/drops/relation_drop.rb

class RelationDrop < Liquid::Drop
  delegate :full_name, :letter_head, to: :relation
  
  attr_reader :relation

  def initialize(relation)
    @relation = relation
  end
  
  def invoices
    relation.invoices.map(&:to_liquid)
  end
  
  def late_invoices
    relation.invoices.late.map(&:to_liquid)
  end
end