python - Django ORM : Selecting related set

Question

假设我有 2 个模型:

class Poll(models.Model):
    category = models.CharField(u"Category", max_length = 64)
    [...]

class Choice(models.Model):
    poll = models.ForeignKey(Poll)
    [...]

给定一个 Poll 对象，我可以查询它的选择:

poll.choice_set.all()

但是，是否有一个实用函数可以从一组 Poll 中查询所有选项？

实际上，我正在寻找类似以下的东西(不支持，我也不寻求它是如何实现的):

polls = Poll.objects.filter(category = 'foo').select_related('choice_set')
for poll in polls:
    print poll.choice_set.all() # this shouldn't perform a SQL query at each iteration

我制作了一个(丑陋的)函数来帮助我实现这一目标:

def qbind(objects, target_name, model, field_name):
    objects = list(objects)
    objects_dict = dict([(object.id, object) for object in objects])
    for foreign in model.objects.filter(**{field_name + '__in': objects_dict.keys()}):
        id = getattr(foreign, field_name + '_id')
        if id in objects_dict:
            object = objects_dict[id]
            if hasattr(object, target_name):
                getattr(object, target_name).append(foreign)
            else:
                setattr(object, target_name, [foreign])
    return objects

具体用法如下:

polls = Poll.objects.filter(category = 'foo')
polls = qbind(polls, 'choices', Choice, 'poll')
# Now, each object in polls have a 'choices' member with the list of choices.
# This was achieved with 2 SQL queries only.

Django 是否已经提供了一些更简单的东西？或者至少，一个片段以更好的方式做同样的事情。

你通常如何处理这个问题？

Answer 1

时间已经过去，现在 Django 1.4 中引入了 prefetch_related()查询集函数。此函数有效地执行建议的 qbind 函数执行的操作。 IE。执行了两个查询，连接发生在 Python 领域，但现在由 ORM 处理。

原来的查询请求现在变成:

polls = Poll.objects.filter(category = 'foo').prefetch_related('choice_set')

如以下代码示例所示，polls QuerySet 可用于获取每个 Poll 的所有 Choice 对象，而无需任何进一步数据库命中:

for poll in polls:
    for choice in poll.choice_set:
        print choice