Determine if an attribute is a `DeferredAttribute` in django

Question

The Context

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I have located a rather critical bug in Django Cache Machine that causes it's invalidation logic to lose its mind after a upgrading from Django 1.4 to 1.7.

The bug is localized to invocations of only() on models that extend cache machine's CachingMixin. It results in deep recursions that occasionally bust the stack, but otherwise create huge flush_lists that cache machine uses for bi-directional invalidation for models in ForeignKey relationships.

class MyModel(CachingMixin):
    id = models.CharField(max_length=50, blank=True)
    nickname = models.CharField(max_length=50, blank=True)
    favorite_color = models.CharField(max_length=50, blank=True)
    content_owner = models.ForeignKey(OtherModel)

---

m = MyModel.objects.only('id').all()

The Bug

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The bug occurs in the following lines(https://github.com/jbalogh/django-cache-machine/blob/f827f05b195ad3fc1b0111131669471d843d631f/caching/base.py#L253-L254). In this case self is a instance of MyModel with a mix of deferred and undeferred attributes:

    fks = dict((f, getattr(self, f.attname)) for f in self._meta.fields
                if isinstance(f, models.ForeignKey))

Cache Machine does bidirectional invalidation across ForeignKey relationships. It does this by looping over all the fields in a Model and storing a series of pointers in cache that point to objects that need invalidated when the object in question is invalidated.

The use of only() in the Django ORM does some meta programming magic that overrides the unfetched attributes with Django's DeferredAttribute implementation. Under normal circumstances an access to favorite_color would invoke DeferredAttribute.__get__(https://github.com/django/django/blob/18f3e79b13947de0bda7c985916d5a04e28936dc/django/db/models/query_utils.py#L121-L146) and fetch the attribute either from the result cache or the data source. It does this by fetching the undeferred representation of the Model in question and calling another only() query on it.

This is the problem when looping over the foreign keys in the Model and accessing their values, Cachine Machine introduces an unintentional recursion. getattr(self, f.attname) on an attribute that is deferred induces a fetch of a Model that has the CachingMixin applied and has deferred attributes. This starts the whole caching process over again.

The Question

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I would like to open a PR to fix this and I believe the answer to this is as simple as skipping over the deferred attributes, but I'm not sure how to do it because accessing the attribute causes the fetch process to start.

If all I have is a handle on an instance of a Model with a mix of deferred and undeferred attributes, Is there a way to determine if an attribute is a DeferredAttribute without accessing it?

    fks = dict((f, getattr(self, f.attname)) for f in self._meta.fields
                if (isinstance(f, models.ForeignKey) and <f's value isn't a Deferred attribute))

Answer 1

Here is how to check if a field is deferred:

from django.db.models.query_utils import DeferredAttribute

is_deferred = isinstance(model_instance.__class__.__dict__.get(field.attname), DeferredAttribute):

Taken from: https://github.com/django/django/blob/1.9.4/django/db/models/base.py#L393