refactoring.rb

def store
  unless @store
    if self.transaction_item && 
      self.transaction_item.packing_slip && 
      self.transaction_item.packing_slip.store_packing_slip &&
      self.transaction_item.packing_slip.store_packing_slip.store

      @store = self.transaction_item.packing_slip.store_packing_slip.store
    end
  end

  @store
end

# becomes

def store
  @store ||= self.transaction_item.try(:packing_slip).try(:store_packing_slip).try(:store)
end