Annotate existing model objects in Django

Question

Is there a way to use something like Django's annotate method, but for a collection of existing model instances instead of a queryset?

Say I have a model like this (all irrelevant details removed):

class Node(Model):
    parent = ForeignKey('self', related_name='children')

If I were fetching some nodes and wanted the child count for each one, I could do this:

nodes = Node.objects.filter(some_filter=True).annotate(child_count=Count('children'))
for node in nodes:
    print(node.child_count)

But what if I already have a collection of Node objects, instead of a queryset? The naïve way of doing this suffers from the N+1 query problem, which is unacceptable for performance:

for node in nodes:
    print(node.children.count()) # executes a separate query for each instance

I essentially want the annotation equivalent of prefetch_related_objects. I'm picturing something like this:

nodes = list(Node.objects.filter(some_filter=True))
annotate_objects(nodes, child_count=Count('children'))
for node in nodes:
    print(node.child_count)

Is there anything like this built into Django? Digging through the docs has not been fruitful for me.

Answer 1

I ended up writing a helper function that implements the API I had imagined:

from collections import defaultdict

def annotate_objects(model_instances, *args, **kwargs):
    """
    The annotation equivalent of `prefetch_related_objects`: works just like the
    queryset `annotate` method, but operates on a sequence of model instances
    instead of a queryset.
    """

    if len(model_instances) == 0:
        return

    # Group instances by model class (since you can hypothetically pass
    # instances of different models).
    instances_by_model_class = defaultdict(list)
    for instance in model_instances:
        instances_by_model_class[type(instance)].append(instance)

    for Model, instances in instances_by_model_class.items():
        pks = set(instance.pk for instance in instances)
        queryset = Model.objects.filter(pk__in=pks).annotate(*args, **kwargs)
        annotation_keys = list(queryset.query.annotations.keys())
        all_annotations = queryset.values(*annotation_keys)
        for instance, annotations in zip(instances, all_annotations):
            for key, value in annotations.items():
                setattr(instance, key, value)

To use:

annotate_objects(nodes, child_count=Count('children'))
for node in nodes:
    print(node.child_count)