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.

Sasha Chedygov · Accepted Answer · 2019-11-26 23:21:23Z

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)

Sasha Chedygov · Accepted Answer · 2019-11-27 00:07:54Z

3

There isn't anything really written into Django for this, unfortunately. The annotation is a specific feature of a queryset.

You may consider adding a @property on your Node model

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

    @property
    def child_count(self)
        return ... #Some logic to return the desired count

Or a workaround I have used before is to just get a queryset from the list I have for example:

nodes = list(Node.objects.filter(some_filter=True)) # This would be your list from somewhere else
node_ids = [n.id for n in nodes]
node_qs = Node.objects.filter(id__in=node_ids).annotate(child_count=Count('children'))

edited Nov 27, 2019 at 0:07

Sasha Chedygov

132k27 gold badges107 silver badges117 bronze badges

answered Nov 26, 2019 at 20:14

Art Vandelay

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2 Comments

Sasha Chedygov Over a year ago

Using a property is a deal-breaker because of the N+1 problem (it would execute a separate count query for every instance instead of a single query for all of them). The workaround is indeed what I do now, it's just overly tedious in some instances to not have that data available directly on the model instances like you would have with annotate. I was hoping Django would have a built-in solution for this, but I guess I could create a helper out of this workaround to achieve the same goal. Thanks!

Art Vandelay Over a year ago

I've had the same frustration many times. This is the easiest solution I've found. Maybe someone else has something better. Good Luck!

xmantas · Accepted Answer · 2021-06-25 13:33:47Z

3

I came up with same workaround as the author, but generalized form.

Simply collect data by making separate queries to avoid N+1, and then assign values for model instances:

instances = Model.objects.filter(...).all()
for instance in instances:
    value = 'custom value'
    setattr(instance, 'new_attribute', value)

then you could simply call new attribute:

instance = instances[0]
print(instance.new_attribute)

outputs custom value.

answered Jun 25, 2021 at 13:33

xmantas

5985 silver badges11 bronze badges

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