python - 在 Django 中注释现有模型对象

Question

有没有办法使用类似 Django 的 annotate 的东西？方法，但对于现有模型实例的集合而不是查询集？

假设我有一个这样的模型(所有不相关的细节都被删除):

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

如果我正在获取一些节点并希望每个节点的子节点计数，我可以这样做:

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

但是如果我已经有了一个 Node 对象集合，而不是一个查询集呢？这样做的天真方式会受到 N+1 query problem 的影响。 ，这对于性能来说是 Not Acceptable :

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

我基本上想要相当于 prefetch_related_objects 的注释.我正在想象这样的事情:

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

Django 中是否内置了类似的东西？浏览文档对我来说并没有取得成果。

Answer 1

我最终编写了一个辅助函数来实现我想象中的 API:

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)

使用:

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