4e22affb24
When releasing the versioned assets work, we stumbled on a problem with old pickled versions of the StaticContent objects residing in cache, which triggered a bug in the code. Not wanting to blow away all cached items, we ended up having to revert and add in some backwards-compatible helper code to ease the transition. With this, we'll now utilize the version argument that Django's caching interface allows, in conjunction with a constant value that can be modified when breaking changes are being made, to let us gracefully ignore older cached course assets.
110 lines
3.3 KiB
Python
110 lines
3.3 KiB
Python
"""
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Core methods
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------------
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.. autofunction:: cache_toolbox.core.get_instance
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.. autofunction:: cache_toolbox.core.delete_instance
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.. autofunction:: cache_toolbox.core.instance_key
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"""
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from django.core.cache import cache
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from django.db import DEFAULT_DB_ALIAS
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from opaque_keys import InvalidKeyError
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from . import app_settings
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def get_instance(model, instance_or_pk, timeout=None, using=None):
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"""
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Returns the ``model`` instance with a primary key of ``instance_or_pk``.
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If the data is cached it will be returned from there, otherwise the regular
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Django ORM is queried for this instance and the data stored in the cache.
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If omitted, the timeout value defaults to
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``settings.CACHE_TOOLBOX_DEFAULT_TIMEOUT`` instead of 0 (zero).
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Example::
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>>> get_instance(User, 1) # Cache miss
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<User: lamby>
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>>> get_instance(User, 1) # Cache hit
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<User: lamby>
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>>> User.objects.get(pk=1) == get_instance(User, 1)
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True
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"""
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pk = getattr(instance_or_pk, 'pk', instance_or_pk)
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key = instance_key(model, instance_or_pk)
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data = cache.get(key)
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if data is not None:
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try:
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# Try and construct instance from dictionary
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instance = model(pk=pk, **data)
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# Ensure instance knows that it already exists in the database,
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# otherwise we will fail any uniqueness checks when saving the
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# instance.
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instance._state.adding = False
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# Specify database so that instance is setup correctly. We don't
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# namespace cached objects by their origin database, however.
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instance._state.db = using or DEFAULT_DB_ALIAS
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return instance
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except:
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# Error when deserialising - remove from the cache; we will
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# fallback and return the underlying instance
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cache.delete(key)
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# Use the default manager so we are never filtered by a .get_queryset()
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# import logging
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# log = logging.getLogger("tracking")
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# log.info( str(pk) )
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instance = model._default_manager.using(using).get(pk=pk)
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data = {}
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for field in instance._meta.fields:
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# Harmless to save, but saves space in the dictionary - we already know
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# the primary key when we lookup
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if field.primary_key:
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continue
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if field.get_internal_type() == 'FileField':
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# Avoid problems with serializing FileFields
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# by only serializing the file name
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file = getattr(instance, field.attname)
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data[field.attname] = file.name
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else:
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data[field.attname] = getattr(instance, field.attname)
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if timeout is None:
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timeout = app_settings.CACHE_TOOLBOX_DEFAULT_TIMEOUT
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cache.set(key, data, timeout)
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return instance
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def delete_instance(model, *instance_or_pk):
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"""
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Purges the cache keys for the instances of this model.
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"""
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cache.delete_many([instance_key(model, x) for x in instance_or_pk])
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def instance_key(model, instance_or_pk):
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"""
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Returns the cache key for this (model, instance) pair.
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"""
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# pylint: disable=protected-access
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return '%s.%s:%d' % (
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model._meta.app_label,
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model._meta.model_name,
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getattr(instance_or_pk, 'pk', instance_or_pk),
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)
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