python - Getting total price for a ManyToManyField in Django -

i'm trying total price manytomanyfield. i'm getting each individual price on price_for_each_item , want total of many items in added 'total_price'

class purchaseorder(models.model):     number_of_products = models.manytomanyfield('product', null =true)     total_price = models.floatfield(verbose_name='total price')  class product(models.model):     products = models.charfield(max_length=256, null =true)     price_for_each_item = models.floatfield(verbose_name = "price")      def __unicode__(self):         return self.products 

for example, if made order of macbook pro =$500, asus computer =$200, , mousepad= $50. i'd want total of go total_price = $750

edit: tried doing this, no avail. have idea i'd have fix work? when try display it says no float object callable

    def get_total_price(self):        return ([p.price_for_each_item p in self.total_price()]) 

moved above.

if understand correctly, retrieve sum object itself. if have order called "order" products assigned (during saving process, should able like:

order.total_price = order.number_of_products.all().aggregate(sum('price_for_each_item')).  

let me know if answers question , i'll move answer.

glad help. not nazi here, please give more thought model field names in future, confusing.

edit re comment:

i'm not sure think overriding model's save method way go work in admin:

def save(self):         super(person, self).save()         self.total_price = self.number_of_products.all().aggregate(sum('price_for_each_item')).          self.save(commit=true) 

haven't tested @ though. have @ this: http://ifacethoughts.net/2009/07/14/calculated-fields-in-django/ if works, accept answer.