OptimisePrime

line_item.rb

class LineItem < ActiveRecord::Base
  belongs_to :order
  has_many :passengers
end

nested.rb

# RUSTY SPOON - BIG-O NO!
def unique_emails(order)
  passengers = []
  
  order.line_items.each do |li|
    li.passengers.each do |pax|
      passengers << pax.email
    end
  end
  
  passengers.uniq!
end

# SLIGHTLY LESS RUSTY SPOON
def unique_emails(order)
  Passenger.joins(line_item: :order)
           .where(orders: { id: order })
           .select(:email)
           .distinct
           .pluck(:email)
end

order.rb

class Order < ActiveRecord::Base
  has_many :line_items
end

passenger.rb

class Passenger < ActiveRecord::Base
  belongs_to :line_item
end