A good query object template

order_lines_from_united_states_query.rb

class OrderLinesFromUnitedStatesQuery
  class << self
    delegate :call, to: :new
  end

  def initialize(relation = OrderLine.all)
    @relation = relation
  end

  def call
    @relation.joins(:order => :billing_address).where('addresses.country' => 'United States')
  end
end

# example usage:
OrderLinesFromUnitedStatesQuery.call.find_each do |ol|
  puts "processing orderline id #{ol.id}"
  # ...
end

# will also allow this:
class OrderLine < ActiveRecord::Base
  scope :from_united_states, OrderLinesFromUnitedStatesQuery
end  

OrderLine.from_united_states.find_each do |ol|
  # processing here
end