Caching query results in django - django

I'm trying to find a way to cache the results of a query that won't change with frequency. For example, categories of products from an e-commerce (cellphones, TV, etc).
I'm thinking of using the template fragment caching, but in this fragment, I will iterate over a list of these categories. This list is avaliable in any part of the site, so it's in my base.html file. Do I have always to send the list of categories when rendering the templates? Or is there a more dynamic way to do this, making the list always available in the template?

Pop your cached query into Django's cache:
from django.core.cache import cache
cache.set('key', queryset)
Then create a context processor to add the value of the cache to all templates:
# myproject/myapp/context_processors.py
from django.core.cache import cache
def cached_queries():
return {'cache', cache.get('key')}
Then add your context processor in your Django settings file:
TEMPLATE_CONTEXT_PROCESSORS += (
'myproject.myapp.context_processors.cached_queries'
)
Now you will be able to access the cache variable in all generic templates and all templates which have a requests context, which a template is given if this is done in the view:
return render_to_response('my_template.html',
my_data_dictionary,
context_instance=RequestContext(request))
When to Set the Cache
It depends on what is contained in the cache. However a common problem is that Django only really gets to execute Python whenever a page request is sent, and this is often not where you want to do this kind of work.
An alternative is to create a custom management command for a particular app. You can then either run this manually when necessary, or more commonly set this to run as a cron job.
To create a management command you must create a class decended from Command inside of a management/commands directory located inside of an app:
# myproject/myapp/management/commands/update_cache.py
from django.core.management.base import NoArgsCommand
from django.core.cache import cache
class Command(NoArgsCommand):
help = 'Refreshes my cache'
def handle_noargs(self, **options):
cache.set('key', queryset)
The name of this file is important as this will be the name of the command. In this case you can now call this on the command line:
python manage.py update_cache

You can also use johnny-cache for automatic caching of querysets. It will (by default) cache all querysets, but you can force it not to cache some.

Related

How to manually clear/update a cached view in django

My goal is to cache a view until an event occurs where the view's cache would need to expire, otherwise cache for 1 hour. This is what I have in urls.py
url(r'^get_some_stuff/$', cache_page(60 * 60, key_prefix='get_some_stuff')(views.StuffView.as_view())),
And this works fine. Now I'm trying to fetch the cached view to verify that there's something there and I tried this:
from django.core.cache import cache
cache.get('get_some_stuff')
But this returns None. I was hoping to do something like this:
from django.core.cache import cache
#relevant event happened
cache.delete('get_some_stuff')
What's the right way to handle cache?
I've tried passing the uri path:
cache.get('/api/get_some_stuff/')
And I still get None returned.
>>> cache.has_key('/api/get_some_stuff/')
False
>>> cache.has_key('/api/get_some_stuff')
False
>>> cache.has_key('get_some_stuff')
False
I've reviewed the suggested answer and it does not solve the underlying issue at all. It does not appear to be as trivial as passing the uri routing path as the key since keys are somewhat abstracted within django.
Here is a code snippet from Relekang about expire #cache_page
from django.core.cache import cache
from django.core.urlresolvers import reverse
from django.http import HttpRequest
from django.utils.cache import get_cache_key
def expire_page_cache(view, args=None):
"""
Removes cache created by cache_page functionality.
Parameters are used as they are in reverse()
"""
if args is None:
path = reverse(view)
else:
path = reverse(view, args=args)
request = HttpRequest()
request.path = path
key = get_cache_key(request)
if cache.has_key(key):
cache.delete(key)
Django's Cache framework only allows to cache data for predefined time and to clear expired cache data you may need to use django-signals to notify some receiver function which clears cache.
And cache.get, cache.has_key, cache.delete requires complete cache_key to be passed not the url or key-prefix. As django takes care of the keys we don't have much control to get or delete data.
If you are using database caching then use raw sql query to delete cache record from the database table when it's stale. write a query which says delete from cache_table with cache_key like ('%1:views.decorators.cache.cache_page%')
I faced same issues with per-view caching and I went with low-level cache api. I cached final result querysets using cache.set() and good part is you can set your own key and play with it.

Django: how to handle caching in views that differ for Ajax requests?

I have a Django application and a postgres backend. It's essentially a search site with a large database, and the data typically changes once a day. I would like to start caching, to reduce load on the database.
I've set up memcached, but I have the following architecture in my views, designed to let my app use Ajax in the front-end:
#cache_page(60 * 60 * 12)
def items(request, pattern=None, specialurl=None):
if request.is_ajax():
template = "result_ajax.html"
else:
template = "index.html"
.. and unfortunately, the combination of caching plus special handling of Ajax calls does not work well.
This is beacuse memcached doesn't distinguish between the Ajax results and the non-Ajax results - so Ajax calls from the front-end are given cached non-Ajax results, and vice versa.
So what I need to do is to figure out how else to cache. I can think of the following options:
Cache only the database queries, for up to a day at a time. Is this possible?
Cache the fragment of the template within result_ajax.html that actually displays the results. (index.html actually includes result_ajax.html.)
Which of these is likely to be the best way to do things?
I would try something like this tell the cache decorator to use a different cache key for Ajax and non-Ajax requests:
from django.views.decorators.cache import cache_page
from django.views.decorators.vary import vary_on_headers
#cache_page(60 * 60 * 12)
#vary_on_headers('X-Requested-With')
def items(request, pattern=None, specialurl=None):
if request.is_ajax():
template = "result_ajax.html"
else:
template = "index.html"

Django View Preprocessing

Just looking for some guidance here on how to implement this concept in "Django fashion".
I am using dbsettings to define site level variables. On each request I want to check one of my site level variables and render a different response based on the value of that variable.
Now I realize I could easily accomplish this just by adding a simple if statement in each of my view functions, but I thought there may be a way I could apply this at a higher level?
Let me know if there is a way to "globalize" this functionality across all requests. Thanks.
Actually, what you really probably want is middleware, if the setting is going to affect the template that's chosen in the first place, or substantially affect the response. It is a good deal more flexible than using a template context processor, which is more appropriate if you simply want to add a couple variables to your context.
You create a middleware.py in your app, which might contain something like:
from django.conf import settings
class MyMiddleware(object):
def process_request(self, request):
request.my_app_setting = settings.MY_APP_SETTING
Don't forget to add your class to your MIDDLEWARE_CLASSES setting.
You can use custom template context processors for passing "global" context to your views.
To accomplish this, create a new contextprocessors.py somewhere in your application with code similar to the example below (all it has to do is return a dict). Then add the path to the file with the function in the TEMPLATE_CONTEXT_PROCESSORS tuple in your settings.py (ie: yourapp.contextprocessors.resource_urls).
from django.conf import settings
def resource_urls(request):
"""Passes global values to templates."""
return dict(
TIME_ZONE = settings.TIME_ZONE,
)
Now you can refer to these keys in your templates as expected: {{ TIME_ZONE }}.

how to pass variables to templates only for some applications

I have a django project, with a lot of applications. Now i need to add some variables to the context dictionary only for some applications.Does django provide any option to achieve this?
I know decorator can be helpful.Is there any other way,like a middleware / context processor that runs automatically but only for specific applications.
A context processor like this can do what you need:
from django.core.urlresolvers import resolve
def app_var(request):
if resolve(request.path).app_name == 'YOUR_APP_NAME':
return {'CUSTOM_VAR': 'VALUE'}
return {}
to install the context processor, put the code in any file you like, and add a entry referencing (e.g. 'folder.file.app_var') it in the CONTEXT_PROCESSORS in your settings.py.

Get user from URL segment with Django

I have a web application which will return a user id based on the first segment of the url, much like Twitter:
http://www.myapplication.com/user-name-goes-here/
It can go deeper too, like so:
http://www.myapplication.com/user-name-goes-here/news/article_1/
In order to break the site down, I am using the following URL routing technique:
(r'^(?P<slug>\w+)/', include('myapp.sites.urls')),
This will then further route the user to the correct page, but as it stands I am having to query the database in every view in order to obtain the user_id based on the first url segment. I was hoping to somehow automate this so I don't have to bloat my views with the same code each time... my solution was to create some middleware which checks the url segment and returns a 404 if its not found:
from django.http import Http404
class DomainMiddleware(object):
def process_request(self, request):
from myapp.sites.models import Sites
dname = request.path.split('/')[1]
if not dname:
return
try:
d = Sites.objects.get(domain__exact=dname)
except Sites.DoesNotExist:
raise Http404
return
This works, but it's trying to parse EVERY request, even those to images, favicons etc.
My question is thus; Is there a way to run this query on every page load without clogging up my views with extra code? If middleware is the solution, how can I modify my code so that it doesn't include EVERY request, only those to successfully routed URLs?
Hope someone can help!
The Django server shouldn't be processing requests for static content URLs - certainly not in production anyway, where you'd have a different web server running to handle that, so this shouldn't be an issue there.
But if you say you'd like this to run for only sucessfully routed URLs, maybe you'd be better of using process_view rather than process_request in your middleware? http://docs.djangoproject.com/en/dev/topics/http/middleware/#process-view
process_view works at view level rather than request level, and provides a view_func argument which you can check so that your code doesn't run when it's the django.views.static.serve view used for serving static media during development.
Whatever happens you should defs be caching that database call if it's going to be used on every view.