Django tips

Note to RG staff: There's also some Django tips found in our private "Tracker" repo's WIKI.

---

How to delete template cache fragment

Navigate to your Django project.

In this example, I want to delete a cache key named "footer".

Fire up the Django python interpreter:

$ python manage.py shell

At the python prompt, run:

>>> import hashlib
>>> from django.core.cache import cache
>>> from django.template import resolve_variable
>>> from django.utils.http import urlquote
>>> args = hashlib.md5(u':'.join([urlquote('')]))
>>> cache_key = 'template.cache.%s.%s' % ('footer', args.hexdigest())
>>> cache.has_key(cache_key)
True
>>> cache.delete(cache_key)
>>>

Done!

---

How to delete template cache fragment (original version)

Kill the template fragment cache:

'''
See https://code.djangoproject.com/browser/django/trunk/django/templatetags/cache.py#L25
First, get the cache key … 
'''

>>> import hashlib
>>> from django.core.cache import cache
>>> from django.template import resolve_variable
>>> from django.utils.http import urlquote

'''
The hashlib argument below can be empty, i.e. ''
'''

>>> args = hashlib.md5(u':'.join([urlquote('RG Index: UO President Richard Lariviere')]))
>>> cache_key = 'template.cache.%s.%s' % ('generic_index_cache', args.hexdigest())
>>> cache.has_key(cache_key)
True
>>> cache.delete(cache_key)

---

Kill a page URL cache

>>> from django.core.cache import cache
>>> from django.http import HttpRequest
>>> from django.utils.cache import _generate_cache_header_key
>>> request = HttpRequest()
>>> key_prefix = ''
>>> request.path = '/topics/lariviere/'
>>> key = _generate_cache_header_key(key_prefix, request)
>>> key
'views.decorators.cache.cache_header..0fed4bf32cb4982c2b50402ea06ca664.en-us'
>>> cache.has_key(key)
True
>>> cache.set(key, None,  0)
>>> cache.has_key(key)
False

---

How to update a Django model

1. Prepare your fixture:

python manage.py dumpdata dox --indent=2 --format=json > ./dox/fixtures/dox.20140520.json

1. Using same code above, backup the fixture that you'll be replacing.

2. Open two terminal windows (we'll call them windows A and B respectively) and log in to same server.

3. In both windows, cd to Django project.

4. In window A, type: python manage.py sqlclear dox (where dox is the app name). Note that sqlreset would also work, but that gives us more info than is needed.

5. In window B, type: python manage.py dbshell; more info on dbshell here.

6. From window A, copy each line's of SQL into window B's database shell prompt (=>); for example:

=> BEGIN;
=> DROP TABLE "dox_file" cascade;
=> ALTER TABLE "dox_page" DROP CONSTRAINT "status_id_refs_id_e05550a5" cascade;
=> DROP TABLE "dox_pagestatus" cascade;
=> DROP TABLE "dox_page" cascade;
=> COMMIT;

**Note:** We've found that it is best to add `cascade` at the end of each line/statement.

Once you commit, the app's tables have been officially dropped (you should get positive feedback from the terminal after each line's execution).

1. Next, in window **A**, run `python manage.py syncdb` to initialize your apps tables (i.e., the ones you just dropped above).

1. Finally, in window **A**, load your fixture: `python manage.py loaddata ./dox/fixtures/dox.20140520.json`

1. In window **A**, touch the wsgi `touch apache/django.wsgi ` to reload the Django instance. Or, you could restart Apache: `sudo service apache2 restart` (Unbuntu).

That's it! You should have fresh data. Check you admin. If something went wrong, load the backup data and/or fix the fixture you were trying to import.

If all goes well, you can `exit` from both windows.