Five class-based views everyone has written by now

Five class-based views everyone has written by now James Aylett artﬁnder.comWednesday, 2 November 11Django 1.3, released in March, introduced CLASS BASED VIEWS, which are intended to bemake writing views easy and delightful. They replaced the function-based generic views, andtherefore must be better, or something.

from django.views.generic import * class AWub(DetailView): model = Wub class Wubs(ListView): model = WubWednesday, 2 November 11We’ll pretend you can’t pass parameters in urlconfs, because that’s icky and also misses thepoint.

from django.views.generic import * class AWub(DetailView): model = Wub class Wubs(ListView): model = Wub def get_queryset(self): return Wub.objects.exclude( hidden=True )Wednesday, 2 November 11The point is that you can override small bits of the ways the views work, to reﬁne them foryour application. Add further context for your templates, change the queryset and so on.There are conventional places to put templates, and for naming template context members.Upshot: little code, big effect.

from django.views.generic import * class NewWub(CreateView): model = Wub class DeleteWub(DeleteView): model = Wub class UpdateWub(UpdateView): model = WubWednesday, 2 November 11Generic views had ways of editing, creating and deleting, so you get that too. Write a suitabletemplate, and EVERYTHING else is automatic, using a default form. If you want a differentform, you can override that easily enough. Sounds good, right?

Wednesday, 2 November 11Unfortunately you are about to enter a world of pain. Let’s consider an example that isn’tcompletely trivial.

from django.db import models class WubFlock(models.Model): name = models.CharField(…) class Wub(models.Model): name = models.CharField(…) flock = models.ForeignKey( WubFlock, related_name=‘wubs’)Wednesday, 2 November 11Okay, so we have a ﬂock of Wubs. We’re building a wub management interface, so we’ll needto create new wubs within a ﬂock. Except…oh no. You can’t do that.

from django.views.generic import * class NewFlockWub(CreateView): model = Wub # what goes here?Wednesday, 2 November 11Okay, so you want to customise the form so it’ll exclude the “flock” field, but set it pre-saveto the flock this wub is going to be in. (Yes you could have a dropdown of all flocks, but thenyour designer will murder you in your sleep.)

from django.views.generic import * class NewFlockWub(CreateView): model = Wub form_class = SomethingSomethingFormWednesday, 2 November 11Okay, so we’ll deﬁne the form somewhere. Only…

from django.views.generic import * class NewFlockWub(CreateView): model = Wub form_class = SomethingSomethingForm # erm, but we need to get # the flock object for the formWednesday, 2 November 11You want to figure out the flock from the URL, say “/flock/my-awesome-wubs”. DetailViewdoes this, and like all class-based views is built up of composable little pieces, so you couldbring in SingleObjectMixin, which provides get_object to do this. Then we could overrideget_form_class to set up the form dynamically. But this behaviour is generic, so…

from moreviews import * class NewFlockWub(BoundCreateView): model = Wub bound_field = ‘flock’ queryset = WubFlock.objects.all()Wednesday, 2 November 11If your BoundCreateView isn’t this easy, you’re doing it wrong. “Bound” because the Wub isbound to the WubFlock.

class DeleteWub(DeleteView): model = Wub class F(forms.ModelForm): class Meta: model = Wub exclude = (‘flock’,) class UpdateWub(UpdateView): model = Wub form_class = FWednesday, 2 November 11We don’t have to worry about binding for deleting, and for updating we just ensure we don’tchange the “ﬂock” ﬁeld.

Wednesday, 2 November 11But what about the WubFlock? We should be able to create it AND ITS WUBS in one go. Intraditional function views you’d do this with forms and formsets within the same request. Sowe want to do the same thing in class-based views.

class ProcessMultipleFormsMixin: """Modify GET and POST behaviour to construct and process multiple forms in one go. Theres always a primary form, which is a ModelForm. By the time secondary forms are saved, self.new_object on the view will contain the primary object, ie the object that the primary form operates on."""Wednesday, 2 November 11This isn’t one of the ﬁve classes, this is just a mixin. It’s not named perfectly, becausealthough it DOES process multiple forms at once, it assumes one is the main form. Thisallows us to save the PRINCIPAL object, and then leaves a reference for all the other forms touse.

from django.views.generic import * class NewFlock(MultiCreateView): model = WubFlock forms_models = [ { ‘model’: Wub, ‘extra’: 1, } ]Wednesday, 2 November 11This syntax is a little opaque, but it didn’t seem worth creating yet more classes just ashelpers when we have lists and dictionaries. The exclude of the project ﬁeld in theModelForm for making little Wubs is taken care of for you.

from django.views.generic import * class UpdateFlock(MultiUpdateView): model = WubFlock forms_models = [ { ‘model’: Wub, ‘extra’: 1, ‘can_delete’: True, } ]Wednesday, 2 November 11extra and can_delete here are both passed through to modelformset_factory. You can also setform inside the dictionary so you don’t just get a default ModelForm but can customise toyour heart’s content.

from django.views.generic import * class UpdateFlock(MultiUpdateView): … def get_forms(self): class WubInlineForm(ModelForm): def save(self): # perhaps we default the # name based on the flock? self.forms_models[0][‘form’] = WubInlineForm return super(...)()Wednesday, 2 November 11You can even dynamically adjust things if you so choose. In fact, you can avoid forms_modelsby implementing get_forms directly if you prefer.

from django.views.generic import * class DeleteFlock(DeleteView): model = WubFlockWednesday, 2 November 11And of course deleting a ﬂock will delete its wubs via the evil of the ORM’s implementingCASCADE DELETE itself.

Wednesday, 2 November 11There’s also a variant which will allow you to create an object bound to another but whichitself has children bound to it. It’s imaginatively called MultiBoundCreateView. That’s not oneof the ﬁve, that’s a bonus one. However now we need to think about non-editing views again,because we’ve still got a problem.

from django.views.generic import * class Wubs(ListView): model = Wub class Flock(DetailView): model = WubFlockWednesday, 2 November 11This is ﬁne until you have a ﬂock with three thousand wubs in. And wubs are very gregarious.

from django.views.generic import * class Wubs(ListView): model = Wub paginate_by = 10 class Flock(DetailView): model = WubFlockWednesday, 2 November 11ListView has pagination built in. Wouldn’t it be nice if we could do that for the CHILDRENbound to the object in a DetailView?

from django.views.generic import * from moreviews import * class Wubs(ListView): model = Wub paginate_by = 10 class Flock(DetailListView): model = WubFlock paginate_by = 10Wednesday, 2 November 11This is the fourth view. Warning: this exists, deep within the Artﬁnder codebase, but isn’t re-usable yet.

Wednesday, 2 November 11We’re not ﬁnished yet.

From: Your Boss <phb@myco.co.com> To: You <peon@myco.co.com> Subject: Permissions Yo dawg. I was over at Big Client Co this morning and noticed that they’re able to edit the WubFlock for the Sinister Government Agency. This contains proprietary Wub technology, and geese, so THIS SHOULD NOT BE ALLOWED.Wednesday, 2 November 11Maybe your site allows everyone to see and do everything. Most don’t. What you used to dowith function views was to check the permission at the top of the view and return a 403. Inclass-based views you need to do that around get_object, which gives you direct access tothe object.

def editable(): def decorator(fn): @wraps(fn, assigned=available_attrs(fn)) def _wrapped_fn(self, *args, **kwargs): obj = fn(self, *args, **kwargs) if obj.editable(self.request.user): return obj else: raise HttpForbidden() return _wrapped_fn return decorator class _UpdateFlock(UpdateFlock): get_object = editable(UpdateFlock.get_object)Wednesday, 2 November 11Oh god oh god I want to die. Ignore that decorators make most people’s heads hurt, justLOOK AT THAT CODE AT THE BOTTOM.

class _UpdateFlock(UpdateFlock): get_object = editable( UpdateFlock.get_object )Wednesday, 2 November 11There is no way this is going to look nice, no matter how many lines we wrap it onto. So thisis the ﬁfth class…that I’m really hoping someone has written. I want to write something likethe following.

class _UpdateFlock(UpdateFlock): def allowed(self): return self.object.editable( self.request.user )Wednesday, 2 November 11Which is basically the same but not ugly. I guess what we actually want here is a permissions-activating mixin.

class _UpdateFlock( UpdateFlock, SingleObjectPermissionsMixin): def allowed(self): return self.object.editable( self.request.user )Wednesday, 2 November 11

class SingleObjectPermissionsMixin( object): def get_object(self): obj = super( SingleObjectPermissionsMixin, self).get_object() if not self.allowed(obj): raise HttpForbidden() return obj def allowed(self): raise NotImplementedError()Wednesday, 2 November 11So this would be the ﬁfth class. I haven’t written this, but I assume someone has. I suspect asI have it here, there are lots of problems, not least that HttpForbidden doesn’t exist in Djangoitself and so you’re dependent on something else to not only provide it but catch it inmiddleware and do something sensible with it.

Summary • BoundCreateView for making Wubs • MultiCreateView for making WubFlocks • MultiUpdateView to update WubFlocks • DetailListView for paginating child objects • SingleObjectPermissionsMixin is mythicalWednesday, 2 November 11The ﬁrst three are written, but Ben caught me by surprise by asking me to talk, so they aren’tpackaged and they’re not directly tested. DetailListView needs extracting from well-usedinternal code. The permissions stuff is entirely speculative; maybe we don’t need it.

Questions? • James Aylett • @jaylett • artﬁnder.com • http://bit.ly/djugl-artWednesday, 2 November 11

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Five class-based views everyone has written by now

by James Aylett

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