I have a custom model manager that looks like this:
class MyManager(models.Manager)
def get_query_set(self):
'''Only get items that are 'approved' and have a `pub_date` that is in
the past. Ignore the rest.'''
queryset = super(MyManager, self).get_query_set()
queryset = queryset.filter(status__in=('a',))
return queryset.filter(pub_date__lte=datetime.utcnow())
And this works well enough; however, I have a problem using Django's generic.list_detail views object_detail and object_list: the queryset seems to be only loading once and, because of this, it isn't fetching the items it should be because, I assume, the utcnow() time has been called only once (when it first loaded).
I assume this is intentional and meant as a performance boost - however, it means that video's show up elsewhere on the site (in places I am not in a object_detail view) before they are available in an object_detail view (see urls.py below). This is leading to 404s ...
Any ideas ? Or do I have to write my own custom views to avoid this ?
Thanks!
urls.py
url(r'^video/(?P<object_id>\d+)$',
list_detail.object_detail,
{ 'queryset': Video.objects.all(), },
name='video_detail',
),
It is not a problem of cache: as you do it now, the queryset definition is evaluated once, while parsing urls, and then, it is never evaluated again.
Solution is actually pretty simple and described in the online documentation: Complex filtering with wrapper functions: just create a small custom view, that will simply call the generic view.
I am actually using a similar solution quite a lot and I feel it quite comfortable.
By the way, a small side note, for this case I would suggest not using a custom manager, and go back instead on a normal filtering.
Try correcting urls.py to:
url(r'^video/(?P<object_id>\d+)$',
list_detail.object_detail,
{ 'queryset': Video.objects.all, }, # here's the difference
name='video_detail',
)
Edit:
If this fail, try apply similar technique(passing callable instead of calling it) to filter():
return queryset.filter(pub_date__lte=datetime.utcnow)
I have an almost identical model Manager to thornomad, and the same problem with generic views.
I have to point out that neither of the suggestions above work:
doing Video.objects.all without parentheses gives an error
doing queryset.filter(pub_date__lte=datetime.utcnow), again without the parentheses, does not give an error but does not fix the problem
I have also tried another way, which is to use a lambda to return the queryset, eg:
qs = lambda *x: Video.objects.all()
url(r'^video/(?P<object_id>\d+)$',
list_detail.object_detail,
{ 'queryset': qs(), },
name='video_detail',
),
...it didn't work either and I can see now I must have been desperate to think it would :)
lazy_qs = lambda *x: lazy(Post.live_objects.all, QuerySet)
blog_posts = {
'queryset': lazy_qs(),
...doesn't work either (gives an error) as utils.functional.lazy doesn't know how to convert the result to a QuerySet properly, as best I can tell.
I think Roberto's answer of wrapping the generic view is the only one that will help.
The django docs should be amended to point out the limitations of the queryset used by generic views (currently the docs have a special note to tell you everything will be okay!)
Related
According to the documentation here: https://djangobook.com/syndication-feed-framework/
If link doesn’t return the domain, the syndication framework will
insert the domain of the current site, according to your SITE_ID
setting
However, I'm trying to generate a feed of magnet: links. The framework doesn't recognize this and attempts to append the SITE_ID, such that the links end up like this (on localhost):
<link>http://localhost:8000magnet:?xt=...</link>
Is there a way to bypass this?
Here's a way to do it with monkey patching, much cleaner.
I like to create a separate folder "django_patches" for these kinds of things:
myproject/django_patches/__init__.py
from django.contrib.syndication import views
from django.contrib.syndication.views import add_domain
def add_domain_if_we_should(domain, url, secure=False):
if url.startswith('magnet:'):
return url
else:
return add_domain(domain, url, secure=False)
views.add_domain = add_domain_if_we_should
Next, add it to your INSTALLED_APPS so that you can patch the function.
settings.py
INSTALLED_APPS = [
'django_overrides',
...
]
This is a bit gnarly, but here's a potential solution if you don't want to give up on the Django framework:
The problem is that the method add_domain is buried deep in a huge method within syndication framework, and I don't see a clean way to override it. Since this method is used for both the feed URL and the feed items, a monkey patch of add_domain would need to consider this.
Django source:
https://github.com/django/django/blob/master/django/contrib/syndication/views.py#L178
Steps:
1: Subclass the Feed class you're using and do a copy-paste override of the huge method get_feed
2: Modify the line:
link = add_domain(
current_site.domain,
self._get_dynamic_attr('item_link', item),
request.is_secure(),
)
To something like:
link = self._get_dynamic_attr('item_link', item)
I did end up digging through the syndication source code and finding no easy way to override it and did some hacky monkey patching. (Unfortunately I did it before I saw the answers posted here, all of which I assume will work about as well as this one)
Here's how I did it:
def item_link(self, item):
# adding http:// means the internal get_feed won't modify it
return "http://"+item.magnet_link
def get_feed(self, obj, request):
# hacky way to bypass the domain handling
feed = super().get_feed(obj, request)
for item in feed.items:
# strip that http:// we added above
item['link'] = item['link'][7:]
return feed
For future readers, this was as of Django 2.0.1. Hopefully in a future patch they allow support for protocols like magnet.
So i want to create:
select * from Post where Post.is_chosen = true order_by create_date
and i want this to occur in the urls.py (that is, not have to define anything, just stick it in the ListView parameters)
How do i do this?
I currently have:
url(r'^$',ListView.as_view(
queryset=Post.objects.get(is_chosen = True).order_by('-pub_date')[:20],
context_object_name='latest_post_list',
template_name='posts/index.html')),
but this has an error - i cannot call order_by on the return object of the "get" This makes sense, but how can i do what i want?
I am open to calling the command from a defined function if it is impossible to do in the url definition!
UPDATE: ARGH I am an idiot.
"get" only returns one item, so of course "order_by" won't work on it. I use filter instead now!
Like the docs say, use .filter() instead.
In each view of my application I need to have navigation menu prepared. So right now in every view I execute complicated query and store the menu in a dictionary which is passed to a template. In templates the variable in which I have the data is surrounded with "cache", so even though the queries are quite costly, it doesn't bother me.
But I don't want to repeat myself in every view. I guessed that the best place to prepare the menu is in my own context processor. And so I did write one, but I noticed that even when I don't use the data from the context processor, the queries used to prepare the menu are executed. Is there a way to "lazy load" such data from CP or do I have to use "low level" cache in CP? Or maybe there's a better solution to my problem?
Django has a SimpleLazyObject. In Django 1.3, this is used by the auth context processor (source code). This makes user available in the template context for every query, but the user is only accessed when the template contains {{ user }}.
You should be able to do something similar in your context processor.
from django.utils.functional import SimpleLazyObject
def my_context_processor(request):
def complicated_query():
do_stuff()
return result
return {
'result': SimpleLazyObject(complicated_query)
If you pass a callable object into the template context, Django will evaluate it when it is used in the template. This provides one simple way to do laziness — just pass in callables:
def my_context_processor(request):
def complicated_query():
do_stuff()
return result
return {'my_info': complicated_query}
The problem with this is it does not memoize the call — if you use it multiple times in a template, complicated_query gets called multiple times.
The fix is to use something like SimpleLazyObject as in the other answer, or to use something like functools.lru_cache:
from functools import lru_cache:
def my_context_processor(request):
#lru_cache()
def complicated_query():
result = do_stuff()
return result
return {'my_info': complicated_query}
You can now use my_info in your template, and it will be evaluated lazily, just once.
Or, if the function already exists, you would do it like this:
from somewhere import complicated_query
def my_context_processor(request):
return {'my_info': lru_cache()(complicated_query)}
I would prefer this method over SimpleLazyObject because the latter can produce some strange bugs sometimes.
(I was the one who originally implemented LazyObject and SimpleLazyObject, and discovered for myself that there is curse on any code artefact labelled simple.)
I would like to know how to show personalized errors with the get_object_or_404 method. I don't want the normal Http404 pages, but I want to display a custom message with the message: the result is none.
Thanks :)
The get_object_or_404() is essentially a simple 5-line function. Unless you have some specific reason for using it, just do:
try:
instance = YourModel.objects.get(pk=something)
except YourModel.DoesNotExist:
return render_to_response('a_template_with_your_error_message.html')
If for whatever reason you have to use get_object_or_404(), you can try putting it in a try: ... except Http404: ... block, but I honestly can't think of a plausible reason for that.
As stated by michael, when using get_object_or_404 you cannot customize the message given on http 404. The message provided in DEBUG does offer information about the exception however: "No MyModel matches the given query."
Check out the doc on this. There are three arguments: Model, *args, and **kwargs. The last two are used to build an argument for either get() or filter() on the Model.
The reason I wrote, however, is to address the question of why we would want to use a helper function such as get_object_or_404() instead of, for example, catching it with an exception like Model.DoesNotExist.
The later solution couples the view layer to the model layer. In an effort to relax this coupling we can take advantage of the controlled coupling offered in the django.shortcuts module[1].
And why exactly aren't you using your server's capeability to do just that?
get_object_or_404() is redirecting to the default 404 page right?
If you are on debug mode you won't see it but when deployed django will just refer to the server's 404 html page.
That can't be done with that shortcut. It will only raise a Http404 exception. Your best bet is a try catch if you want full control. Eg.
try:
obj = Model.objects.get(pk = foo)
except:
return HttpResponseRedirect('/no/foo/for/you')
#or
return render_to_response ...
I come from a Cake background, and I'm just starting to learn Django now. I'm liking it quite a bit, but I kinda wish it used convention over configuration like cake does. So,
How can I get Cake-style URLs automatically? For example, if I went to mysite.com/posts/view/5 it would load up mysite.posts.views.view and pass an argument 5 to it? I was thinking I could add something like (r'^(.*)/(.*)', 'mysite.$1.$2'), to urls.py, but of course, that won't work.
How can I automatically load up a template? Each view function should automatically load a template like templates/posts/view.html.
Is this even possible, or do I have to hack the core of Django?
Here's my solution, based on what Carl suggested:
urlpatterns = patterns('',
# url pats here
url(r'^(?P<app>\w+)/(?P<view>\w+)/(?P<args>.*)$', 'urls.dispatch')
)
def dispatch(req, app, view, args): # FIXME: ignores decorators on view func!
func = get_callable(app+'.views.'+view)
if args:
ret = func(req, *args.split('/'))
else:
ret = func(req)
if type(ret) is dict:
return render_to_response(app+'/'+view+'.html', ret)
else:
return ret
Seems to be working pretty well with initial tests. Solves both problems with a single function. Probably won't support GET-style arguments tho.
Those points are both implementable without hacking Django core, but either one will require a non-trivial level of familiarity with advanced Python techniques.
You can do the generic URL pattern with a pattern like this:
url(r'^(?P<appname>\w+)/(?P<viewfunc>\w+)/(?P<args>.*)$', 'myresolverfunc')
Then define a 'myresolverfunc' "view" function that takes "appname", "viewfunc", and "args" parameters, and implement whatever logic you want, splitting args on "/" and dynamically importing and dispatching to whatever view function is referenced. The trickiest part is the dynamic import, you can search Django's source for "importlib" to see how dynamic imports are done internally various places.
The automatic template loader can be implemented as a view function decorator similar to the various "render_to" decorators out there, except you'll generate the template name rather than passing it in to the decorator. You'll have to introspect the function object to get its name. Getting the app name will be trickier; you'll probably just want to hardcode it as a module-level global in each views.py file, or else work in conjunction with the above URL dispatcher, and have it annotate the request object with the app name or some such.
I don't you'll need to hack the core of Django for this. It sounds like you might be in need of generic views. Also check out the Generic Views topic guide.
The first example given in the generic views documentation sounds like your first bullet point:
Example:
Given the following URL patterns:
urlpatterns = patterns('django.views.generic.simple',
(r'^foo/$', 'direct_to_template', {'template':'foo_index.html'}),
(r'^foo/(?P<id>\d+)/$', 'direct_to_template', {'template':'foo_detail.html'}),
)
... a request to /foo/ would render the template foo_index.html, and a request to /foo/15/ would render the foo_detail.html with a context variable {{ params.id }} that is set to 15.