python - Tricky Django QuerySet with Complicated ForeignKey Traversals -

i adapting jim mcgaw's e-commerce site beginning django e-commerce client's use. client sells builds of computers composed of custom set of parts. parts go system change. instance, super samurai system sold 2 years have different parts 1 sell tomorrow. sell each super samurai system need snapshot of parts went super samurai @ moment in time of sale.

i having problem queryset copies of parts table maps parts builds (i.e. parts go super samurai)...

class build(models.model):     build = models.foreignkey(partmodel, related_name='+')     part = models.foreignkey(partmodel, related_name='+')     quantity = models.positivesmallintegerfield(default=1)      class meta:         abstract = true         unique_together = ('build', 'part')      def __unicode__(self):         return self.build.name + ' ' + str(self.quantity) + ' * ' + \                self.part.family.make.name + ' ' + self.part.name  class buildpart(build):     pass      class meta:         verbose_name = "build part" 

i need copy build parts buildpart table orderbuildpart table...

class orderbuildpart(build):     orderitem = models.foreignkey(orderitem, unique=false)      class meta:         verbose_name = "ordered build part" 

...so in future know parts went so-and-so's build.

mcgaw's e-commrece site doesn't allow items bundles of other items. rather create nightmarish scenario of 2 different tables (and 2 series of skus) builds , parts, wanted build other part...

class partmodel(models.model):     family = models.foreignkey(partfamily)     name = models.charfield("model name", max_length=50, unique=true)     slug = models.slugfield(help_text="http://www.knowele.com/<b>*slug*</b>",                             unique=true)     vpn = models.charfield("vpn", help_text="vendor's part number",                                max_length=30, blank=true, null=true)     url = models.urlfield("url", blank=true, null=true)     costurl = models.urlfield("cost url", blank=true, null=true)     cost = models.decimalfield(help_text="how knowele.com pays", max_digits=9, decimal_places=2, blank=true, null=true)     price = models.decimalfield(help_text="how customer pays", max_digits=9, decimal_places=2, blank=true, null=true)     isactive = models.booleanfield(default=true)     isbestseller = models.booleanfield(default=false)     isfeatured = models.booleanfield(default=false)     isbuild = models.booleanfield(default=false)     description = models.textfield(blank=true, null=true)     updated = models.datetimefield(auto_now=true)     created = models.datetimefield(auto_now_add=true)     buildpart = models.manytomanyfield('self', through='buildpart',                                 symmetrical=false, related_name='+')      class meta:         ordering = ['name']         verbose_name = "product model"      def __unicode__(self):         return self.name      def get_absolute_url(self):         django.core.urlresolvers import reverse         return reverse('productdetail', args=[self.slug]) 

the buildpart field references manytomany buildpart table allows build have many parts , part associated many builds.

through adapting mcgaw's code pretty need until finalize paypal payment , try record parts went sold builds @ precise moment of sale...

def payment(request):     token = request.post['token']     payer = request.post['payer']      result = paypal.do_express_checkout_payment(request, token, payer)      if result['ack'][0] in ['success', 'successwithwarning']:         cart = cart.objects.get(cart_id=get_cart_id(request))         finalorder = order()         finalorder.cart_id = get_cart_id(request)         finalorder.token = token         finalorder.corid = result['correlationid'][0]         finalorder.payerid = payer         finalorder.ipaddress = request.meta['remote_addr']         finalorder.first = cart.first         finalorder.last = cart.last         finalorder.address = cart.address         finalorder.email = cart.email         finalorder.transactionid = result['paymentinfo_0_transactionid'][0]         finalorder.status = 'f'         finalorder.save()          item in get_cart_items(request):             oi = orderitem()             oi.cart_id = item.cart_id             oi.quantity = item.quantity             oi.product = item.product             oi.price = item.price()             oi.save()              if item.product.isbuild:                 part in get_build_parts(request, item):                     bp = orderbuildpart()                     bp.build = part.build                     bp.part = part.part                     bp.quantity = part.quantity                     bp.orderitem = oi                     bp.save()          empty_cart(request)          return render(request, 'payment.html', locals()) 

everything seems fine until hit get_build_parts function...

def get_build_parts(request, part):     return buildpart.objects.filter(build__id=part__product__pk) 

...where django's post-mortem complains "nameerror @ /payment/ global name 'part__product__pk' not defined"

how traverse these complicated relationships boss can parts went each customer's builds?

the value side of lookup doesn't work way think does. double-underscore stuff left-hand side only: in effect, it's hack round python's syntax requirements. on right-hand side, pass normal expression, can follow object relationships using standard dot syntax:

return buildpart.objects.filter(build__id=part.product.pk)