123 lines
3.4 KiB
Python
123 lines
3.4 KiB
Python
"""
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Caching instances via ``related_name``
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--------------------------------------
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``cache_relation`` adds utility methods to a model to obtain ``related_name``
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instances via the cache.
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Usage
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~~~~~
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::
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from django.db import models
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from django.contrib.auth.models import User
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class Foo(models.Model):
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user = models.OneToOneField(
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User,
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primary_key=True,
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related_name='foo',
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)
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name = models.CharField(max_length=20)
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cache_relation(User.foo)
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::
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>>> user = User.objects.get(pk=1)
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>>> user.foo_cache # Cache miss - hits the database
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<Foo: >
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>>> user = User.objects.get(pk=1)
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>>> user.foo_cache # Cache hit - no database access
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<Foo: >
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>>> user = User.objects.get(pk=2)
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>>> user.foo # Regular lookup - hits the database
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<Foo: >
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>>> user.foo_cache # Special-case: Will not hit cache or database.
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<Foo: >
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Accessing ``user_instance.foo_cache`` (note the "_cache" suffix) will now
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obtain the related ``Foo`` instance via the cache. Accessing the original
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``user_instance.foo`` attribute will perform the lookup as normal.
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Invalidation
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~~~~~~~~~~~~
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Upon saving (or deleting) the instance, the cache is cleared. For example::
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>>> user = User.objects.get(pk=1)
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>>> foo = user.foo_cache # (Assume cache hit from previous session)
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>>> foo.name = "New name"
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>>> foo.save() # Cache is cleared on save
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>>> user = User.objects.get(pk=1)
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>>> user.foo_cache # Cache miss.
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<Foo: >
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Manual invalidation may also be performed using the following methods::
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>>> user_instance.foo_cache_clear()
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>>> User.foo_cache_clear_fk(user_instance_pk)
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Manual invalidation is required if you use ``.update()`` methods which the
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``post_save`` and ``post_delete`` hooks cannot intercept.
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Support
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~~~~~~~
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``cache_relation`` currently only works with ``OneToOneField`` fields. Support
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for regular ``ForeignKey`` fields is planned.
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"""
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from django.db.models.signals import post_save, post_delete
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from .core import get_instance, delete_instance
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def cache_relation(descriptor, timeout=None):
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rel = descriptor.related
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related_name = '%s_cache' % rel.field.related_query_name()
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@property
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def get(self):
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# Always use the cached "real" instance if available
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try:
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return getattr(self, descriptor.cache_name)
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except AttributeError:
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pass
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# Lookup cached instance
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try:
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return getattr(self, '_%s_cache' % related_name)
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except AttributeError:
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pass
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# import logging
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# log = logging.getLogger("tracking")
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# log.info( "DEBUG: "+str(str(rel.model)+"/"+str(self.pk) ))
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instance = get_instance(rel.model, self.pk, timeout)
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setattr(self, '_%s_cache' % related_name, instance)
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return instance
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setattr(rel.parent_model, related_name, get)
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# Clearing cache
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def clear(self):
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delete_instance(rel.model, self)
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@classmethod
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def clear_pk(cls, *instances_or_pk):
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delete_instance(rel.model, *instances_or_pk)
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def clear_cache(sender, instance, *args, **kwargs):
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delete_instance(rel.model, instance)
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setattr(rel.parent_model, '%s_clear' % related_name, clear)
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setattr(rel.parent_model, '%s_clear_pk' % related_name, clear_pk)
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post_save.connect(clear_cache, sender=rel.model, weak=False)
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post_delete.connect(clear_cache, sender=rel.model, weak=False)
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