NewsBlur/utils/cache/base.py

from django.db.models.manager import Manager
from django.db.models.base import ModelBase, Model
from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
from django.db.models.fields import FieldDoesNotExist
from django.db.models.options import Options
from django.db.models import signals
from django.db.models.loading import register_models, get_model
from django.dispatch import dispatcher
from django.utils.functional import curry
from django.conf import settings

from django.core.cache import cache

import types
import sys

from manager import CacheManager
from utils import get_cache_key_for_pk

DEFAULT_CACHE_TIME = 60*60*60 # the maximum an item should be in the cache

# Signals rundown:
# .cache(expire_on=['create', 'update', 'delete'])
# use namespaces possible so the cache key becomes key_name:expire_namespace(not always present):hash

# for example, a call with no expires:
# db_table:hash

# a call with a delete expires
# db_table:0,0,0:hash

# the numbers represent our current namespace level for the 3 expiration methods
# in order to do this, we'd have to actually store another cache key per model
# and to support threading, query that cache key everytime we do any cache queryset
# hit
# e.g. cache.get('ns:db_table') = 0,0,0

# when a new row is created, we'd set that to 1,0,0
# which would invalidate anything that had a create expiration set because the key is
# now invalid, because the namespace changed.

# if you only had create expirations set
# your namespace would be :0: -- its all about the queryset call, you still have to
# call it the same way throughout your code

# We can also add a table namespace, which says "delete everything" so our
# cache key now becomes db_table:ns_count:0,0,0:hash
# where the 0,0,0: is optional

# ns_count would be stored in the same ns:db_table key and starts at 0
# this would most likely only be incremented if you did a push to your site
# and needed to say wipe all articles because the dataset changed.

class CachedModelBase(ModelBase):
    # TODO: find a way to not overwrite __new__ like this
    def __new__(cls, name, bases, attrs):
        # If this isn't a subclass of CachedModel, don't do anything special.
        try:
            if not filter(lambda b: issubclass(b, CachedModel), bases):
                return super(CachedModelBase, cls).__new__(cls, name, bases, attrs)
        except NameError:
            # 'CachedModel' isn't defined yet, meaning we're looking at Django's own
            # Model class, defined below.
            return super(CachedModelBase, cls).__new__(cls, name, bases, attrs)

        # Create the class.
        new_class = type.__new__(cls, name, bases, {'__module__': attrs.pop('__module__')})
        new_class.add_to_class('_meta', Options(attrs.pop('Meta', None)))
        new_class.add_to_class('DoesNotExist', types.ClassType('DoesNotExist', (ObjectDoesNotExist,), {}))
        new_class.add_to_class('MultipleObjectsReturned',
            types.ClassType('MultipleObjectsReturned', (MultipleObjectsReturned, ), {}))

        # Build complete list of parents
        for base in bases:
            # TODO: Checking for the presence of '_meta' is hackish.
            if '_meta' in dir(base):
                new_class._meta.parents.append(base)
                new_class._meta.parents.extend(base._meta.parents)


        if getattr(new_class._meta, 'app_label', None) is None:
            # Figure out the app_label by looking one level up.
            # For 'django.contrib.sites.models', this would be 'sites'.
            model_module = sys.modules[new_class.__module__]
            new_class._meta.app_label = model_module.__name__.split('.')[-2]

        # Bail out early if we have already created this class.
        m = get_model(new_class._meta.app_label, name, False)
        if m is not None:
            return m

        # Add all attributes to the class.
        for obj_name, obj in attrs.items():
            new_class.add_to_class(obj_name, obj)

        # Add Fields inherited from parents
        for parent in new_class._meta.parents:
            for field in parent._meta.fields:
                # Only add parent fields if they aren't defined for this class.
                try:
                    new_class._meta.get_field(field.name)
                except FieldDoesNotExist:
                    field.contribute_to_class(new_class, field.name)

        new_class._prepare()

        register_models(new_class._meta.app_label, new_class)
        # Because of the way imports happen (recursively), we may or may not be
        # the first class for this model to register with the framework. There
        # should only be one class for each model, so we must always return the
        # registered version.
        return get_model(new_class._meta.app_label, name, False)

class CachedModel(Model):
    """
    docstring for CachedModel
    """
    __metaclass__ = CachedModelBase

#    objects = CacheManager()
#    nocache = Manager()
    
    # Maybe this would work?
    @classmethod
    def _prepare(cls):
        # TODO: How do we extend the parent classes classmethod properly?
        # super(CachedModel, cls)._prepare() errors
        opts = cls._meta
        opts._prepare(cls)

        if opts.order_with_respect_to:
            cls.get_next_in_order = curry(cls._get_next_or_previous_in_order, is_next=True)
            cls.get_previous_in_order = curry(cls._get_next_or_previous_in_order, is_next=False)
            setattr(opts.order_with_respect_to.rel.to, 'get_%s_order' % cls.__name__.lower(), curry(method_get_order, cls))
            setattr(opts.order_with_respect_to.rel.to, 'set_%s_order' % cls.__name__.lower(), curry(method_set_order, cls))

        # Give the class a docstring -- its definition.
        if cls.__doc__ is None:
            cls.__doc__ = "%s(%s)" % (cls.__name__, ", ".join([f.attname for f in opts.fields]))

        if hasattr(cls, 'get_absolute_url'):
            cls.get_absolute_url = curry(get_absolute_url, opts, cls.get_absolute_url)

        cls.add_to_class('objects', CacheManager())
        cls.add_to_class('nocache', Manager())
        cls.add_to_class('_default_manager', cls.nocache)
        dispatcher.send(signal=signals.class_prepared, sender=cls)
    
    @staticmethod
    def _get_cache_key_for_pk(model, pk):
        return get_cache_key_for_pk(model, pk)
    
    @property
    def cache_key(self):
        return self._get_cache_key_for_pk(self.__class__, self.pk)
    
    def save(self, *args, **kwargs):
        cache.set(self._get_cache_key_for_pk(self.__class__, self.pk), self)
        super(CachedModel, self).save(*args, **kwargs)

    def delete(self, *args, **kwargs):
        # TODO: create an option that tells the model whether or not it should
        # do a cache.delete when the object is deleted. For memcached we
        # wouldn't care about deleting.
        cache.delete(self._get_cache_key_for_pk(self.__class__, self.pk))
        super(CachedModel, self).delete(*args, **kwargs)