有没有办法检查相关对象是否已经被获取？ [英] Is there a way to check whether a related object is already fetched?

问题描述

我希望能够使用 select_related 或 prefetch_related ，以便我可以相应地序列化数据。下面是一个示例：

I would like to be able to check if a related object has already been fetched by using either select_related or prefetch_related, so that I can serialize the data accordingly. Here is an example:

class Address(models.Model):
    street = models.CharField(max_length=100)
    zip = models.CharField(max_length=10)

class Person(models.Model):
    name = models.CharField(max_length=20)
    address = models.ForeignKey(Address)

def serialize_address(address):
    return {
        "id": address.id,
        "street": address.street,
        "zip": address.zip
    }

def serialize_person(person):
    result = {
        "id": person.id,
        "name": person.name
    }
    if is_fetched(person.address):
        result["address"] = serialize_address(person.address)
    else:
        result["address"] = None

######

person_a = Person.objects.select_related("address").get(id=1)
person_b = Person.objects.get(id=2)

serialize_person(person_a) #should be object with id, name and address
serialize_person(person_b) #should be object with only id and name

在此示例中，函数 is_fetched 是我要的东西。我想确定person对象是否已经有一个解析地址，并且只有在有解析地址的情况下，也应该对其进行序列化。

In this example, the function is_fetched is what I am looking for. I would like to determine if the person object already has a resolves address and only if it has, it should be serialized as well. But if it doesn't, no further database query should be executed.

那么在Django中有没有办法做到这一点？

So is there a way to achieve this in Django?

推荐答案

如果已获取 address 关系，则Person对象将具有称为<$ c的填充属性。 $ c> _address_cache ;您可以检查一下。

If the address relation has been fetched, then the Person object will have a populated attribute called _address_cache; you can check this.

def is_fetched(obj, relation_name):
    cache_name = '_{}_cache'.format(relation_name)
    return getattr(obj, cache_name, False)

请注意需要使用对象和关系名称来调用它：

Note you'd need to call this with the object and the name of the relation:

is_fetched(person, 'address')

因为执行 person.address 会立即触发获取。

since doing person.address would trigger the fetch immediately.

编辑反向或多对多关系只能通过 prefetch_related 获取；填充单个属性 _prefetched_objects_cache ，该属性是列表的字典，其中键是相关模型的名称。例如，如果您这样做：

Edit reverse or many-to-many relations can only be fetched by prefetch_related; that populates a single attribute, _prefetched_objects_cache, which is a dict of lists where the key is the name of the related model. Eg if you do:

addresses = Address.objects.prefetch_related('person_set')

，那么地址中的每个项目都会具有 _prefetched_objects_cache 字典包含一个个人 键。

then each item in addresses will have a _prefetched_objects_cache dict containing a "person' key.

请注意，这两个都是下划线属性，这意味着它们是私有API的一部分；您可以自由使用它们，但Django也可以在将来的版本中自由更改它们。

Note, both of these are single-underscore attributes which means they are part of the private API; you're free to use them, but Django is also free to change them in future releases.

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