payment.rb

class Payment < ActiveRecord::Base
  def self.filtered(restrictions)
    PaymentFilter.new(self.scoped).restrict(restrictions)
  end

  def self.year(year)
    where(:year => year)
  end

  def self.person_id(person_id)
    where(:person_id => person_id)
  end

  def self.taxable(taxable_type, taxable_id)
    where(:taxable_type => taxable_type, :taxable_id => taxable_id)
  end
end

class PaymentFilter
  def initialize(relation)
    self.relation = relation
  end

  def restrict(restrictions)
    year! restrictions[:year] if restrictions[:year]
    person_id! restrictions[:person_id] if restrictions[:person_id]
    taxable! restrictions[:taxable_type], restrictions[:taxable_id] if restrictions[:taxable_type] && restrictions[:taxable_id]

    relation
  end

  protected

  attr_accessor :relation

  def year!(year)
    self.relation = relation.year(year)
  end

  def person_id!(person_id)
    self.relation = relation.person_id(person_id)
  end

  def taxable!(taxable_type, taxable_id)
    self.relation = relation.taxable(taxable_type, taxable_id)
  end
end

restrictions = { :year => '1998', :person_id => 37, :taxable_type => 'Property', :taxable_id => 42 }
payments = Payment.filtered(restrictions)

payments.each do |payment|
  puts payment.year
  puts payment.person
  puts payment.taxable
end

No exemplo do site, o filtro tem uma espécie de delegate do where para a relation. Poderia fazer isso para fazer o where direto no filtro e eliminar os métodos do modelo =D

No site, o autor incluiu esse método na classe do filter:

def where(*a)
  @relation = @relation.where(*a)
end

Eu acho que seria mais simples só colocar um delegate pro relation assim:

# attr_reader :relation # já tem o attr_accessor no exemplo
delegate :where, :to => :relation

De qualquer forma, a ideia é a mesma do autor do post, chamar o where no próprio filter.

UPDATE

Faltou como o código de um dos filters ficaria:

def year!(year)
  self.relation = where(:year => year)
end