Django tips and_tricks (1)

Aug 26, 2010Download as PPTX, PDF2 likes1,155 views

This document contains tips and tricks for working with Django, including debugging production issues, handling foreign key deletions, resolving concurrency issues with counter fields, and simplifying atomic updates. Some tips include adding a debug middleware, using nullable foreign keys, Django F expressions, bulk updates, and a custom update method. The document provides code examples and explanations for various common Django problems and solutions.

Django Tips and Tricks@andymccurdyhttp://github.com/andymccurdy/django-tips-and-tricks

Debugging Production# site.wsgifrom whiskey.core import StaffDebugWSGIHandlerapplication = handlers.StaffDebugWSGIHandler()

Main Ticket: #7539First Reported: 6/25/2008 !!!Duplicate Tickets: #10262, #6870, #12382, #12166

Concurrency Issuesdef like(request, pk): m = MyObject.objects.get(pk=pk) m.likes += 1 m.save() return HttpResponse('')What happens if two requests come in at the same time?

Django "F" ExpressionsNew in Django 1.1from django.db.models import Fdef like(request, pk): m = MyObject.objects.get(pk=pk) m.likes = F('likes') + 1 m.save() return HttpResponse('')

.save() Updates All Fieldsfrom django.db.models import Fdef like(request, pk): m = MyObject.objects.get(pk=pk) m.likes = F('likes') + 1 m.save() # race condition with "hates" return HttpResponse('')def hate(request, pk): m = MyObject.objects.get(pk=pk) m.hates = F('hates') + 1 m.save() # race condition with "likes" return HttpResponse('')

QuerySet.update()from django.db.models import Fdef like(request, pk):MyObject.objects.filter(pk=pk).update( likes=F('likes') + 1) return HttpResponse('')def hate(request, pk):MyObject.objects.filter(pk=pk).update( likes=F('hates') + 1) return HttpResponse('')That's a lot of typing.What about ORM cache invalidation?

Even Easierfrom whiskey.core import updatedef like(request, pk): m = MyObject.objects.get(pk=pk)update(m, likes=F('likes') + 1) return HttpResponse('')def hate(request, pk): m = MyObject.objects.get(pk=pk)update(m, hates=F('hates') + 1) return HttpResponse('')

Thanks!Questions?http://github.com/andymccurdy/django-tips-and-tricks@andymccurdyandy@whiskeymedia.comWe're Hiring!

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Django tips and_tricks (1)

1. Django Tips and Tricks@andymccurdyhttp://github.com/andymccurdy/django-tips-and-tricks

2. Something Went Wrong

3. Debugging Production# site.wsgifrom whiskey.core import StaffDebugWSGIHandlerapplication = handlers.StaffDebugWSGIHandler()

4. Something Useful

5. Foreign Key Woesclass Profile(models.Model): name = models.CharField(max_length=64)class Image(models.Model): profile = models.ForeignKey(Profile)>>> p = Profile.objects.create(name='andy')>>> i = Image.objects.create(profile=p)>>> p.delete()>>> Image.objects.filter(profile=p)[]

6. Foreign Key Woesclass Profile(models.Model): name = models.CharField(max_length=64) icon = models.ForeignKey(Image, null=True)class Image(models.Model): pass>>> i = Image.objects.create()>>> p = Profile.objects.create(name='andy', icon=i)>>> i.delete()>>> Profile.objects.filter(name='andy')[] # WTF ?!?!?!?!?!?

8. Main Ticket: #7539First Reported: 6/25/2008 !!!Duplicate Tickets: #10262, #6870, #12382, #12166

9. NullableForeignKeyfrom whiskey.core import NullableForeignKeyclass Profile(models.Model): name = models.CharField(max_length=64) icon = NullableForeignKey(Image)class Image(models.Model): pass>>> i = Image.objects.create()>>> p = Profile.objects.create(name='andy', icon=i)>>> i.delete()>>> Profile.objects.filter(name='andy')>>> [<Profile: Profile object>]>>> print Profile.objects.filter(name='andy')[0].iconNone

10. Concurrency Issuesdef like(request, pk): m = MyObject.objects.get(pk=pk) m.likes += 1 m.save() return HttpResponse('')What happens if two requests come in at the same time?

11. Django "F" ExpressionsNew in Django 1.1from django.db.models import Fdef like(request, pk): m = MyObject.objects.get(pk=pk) m.likes = F('likes') + 1 m.save() return HttpResponse('')

12. .save() Updates All Fieldsfrom django.db.models import Fdef like(request, pk): m = MyObject.objects.get(pk=pk) m.likes = F('likes') + 1 m.save() # race condition with "hates" return HttpResponse('')def hate(request, pk): m = MyObject.objects.get(pk=pk) m.hates = F('hates') + 1 m.save() # race condition with "likes" return HttpResponse('')

13. QuerySet.update()from django.db.models import Fdef like(request, pk):MyObject.objects.filter(pk=pk).update( likes=F('likes') + 1) return HttpResponse('')def hate(request, pk):MyObject.objects.filter(pk=pk).update( likes=F('hates') + 1) return HttpResponse('')That's a lot of typing.What about ORM cache invalidation?

14. Even Easierfrom whiskey.core import updatedef like(request, pk): m = MyObject.objects.get(pk=pk)update(m, likes=F('likes') + 1) return HttpResponse('')def hate(request, pk): m = MyObject.objects.get(pk=pk)update(m, hates=F('hates') + 1) return HttpResponse('')

15. Thanks!Questions?http://github.com/andymccurdy/django-tips-and-tricks@andymccurdyandy@whiskeymedia.comWe're Hiring!

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