Cached ReferenceProperty

Piece of cake

Earlier I wrote about my wish to subclass ReferenceProperty so the collection would not be fetched every time I iterate though it. Well, it was so easy I can post the whole implementation here.

from google.appengine.ext import db

class CachedReferenceProperty(db.ReferenceProperty):

  def __property_config__(self, model_class, property_name):
    super(CachedReferenceProperty, self).__property_config__(model_class,
                                                       property_name)
    #Just carelessly override what super made
    setattr(self.reference_class,
            self.collection_name,
            _CachedReverseReferenceProperty(model_class, property_name,
                self.collection_name))

class _CachedReverseReferenceProperty(db._ReverseReferenceProperty):

    def __init__(self, model, prop, collection_name):
        super(_CachedReverseReferenceProperty, self).__init__(model, prop)
        self.__collection_name = collection_name

    def __get__(self, model_instance, model_class):
        if model_instance is None:
            return self
        if self.__collection_name in model_instance.__dict__:# why does it get here at all?
            return model_instance.__dict__[self.__collection_name]

        query=super(_CachedReverseReferenceProperty, self).__get__(model_instance,
            model_class)
        #replace the attribute on the instance
        res=[c for c in query]
        model_instance.__dict__[self.__collection_name]=res
        return res

    def __delete__ (self, model_instance):
        if model_instance is not None:
            del model_instance.__dict__[self.__collection_name]

Having these classes now we can rewrite previous example as:

class Master(db.Model):
  pass

class Detail(db.Model):
  master=CachedReferenceProperty(Master)

Try to run the same cycle and you will see it executes instantly even with 100,000 iterations instead of 1000.

Is it a free cake?

Not exactly. Try this:

m=Master()
m.put()
d1=Detail(master=m)
d1.put()
print m.detail_set
d2=Detail(master=m)
d2.put()
print m.detail_set

The second time it returned a wrong result, which did not include d2. So we need a way to reset the cached value and fetch up-to-date values from the datastore. Fortunately, it's achieved easily:

del m.detail_set
print m.detail_set

This is why I implemented _CachedReverseReferenceProperty.__delete__. When m.__dict__ has no key'detail_set', m.detail_set is dispatched to type(m).__dict__('detail_set'), and there I call the base class to access the datastore. What surprised me is when I do have m.__dict__('detail_set'), m.detail_set is still dispatched to Master.__dict__('detail_set'). I don't understand why that happens, so I worked around this problem. Have to learn Python better to answer that question.