优化django查询拉外键和django-taggit关系

2022-01-02 00:00:00 django django-templates mysql django-taggit

我有一个如下定义的待办事项模型:

I have a todo model defined below:

class Action(models.Model):
    name = models.CharField("Action Name", max_length=200, unique = True)

    complete = models.BooleanField(default=False, verbose_name="Complete?")

    reoccurance = models.ForeignKey(Reoccurance, blank=True, null=True, verbose_name="Reoccurance")
    notes = models.TextField("Notes", blank=True)

    tags = TaggableManager()

class Reoccurance(models.Model):
    label = models.CharField("Label", max_length=50, unique = True)
    days = models.IntegerField("Days")

我想列出所有未完成的操作:

I want to list all of the actions that are incomplete:

actions = Action.objects.filter(complete=False)

我的动作列表模板循环:

My template loops of the actions list:

{% for action in actions %}
    <p>{{ action }}</p>
    {% if action.reoccurance %}
        <p>{{ action.reoccurance }}</p>
    {% endif %}
    {% for tag in action.tags.all %}
        <span>{{ tag }}</span>{% if not forloop.last %}, {% endif %}
    {% endfor %}
{% endfor %}

使用 django-debug-toolbar，我看到每个动作，我在 {% if action.reoccurance %} 和 {% for tag in action.tags.all %} 上访问数据库.

Using django-debug-toolbar, I see that for every action, I'm hitting the database on {% if action.reoccurance %} and {% for tag in action.tags.all %}.

是否有更好的方法来编写我的查询，以便在每次循环迭代时都不会 ping 数据库?我认为它与 select_related 有关，但我不确定如何处理 django-taggit.

Is there a better way to write my query so that the database isn't pinged for every iteration of the loop? I think it has something to do with select_related, but I'm not sure what to do about django-taggit.

更新 我得到了部分答案.select_related 确实有效，但我必须指定 reoccurance，可能是因为我不能将它用于标签:

Update I got part of my answer. select_related does work, but I had to specify reoccurance, probably because I can't use it for tags:

actions = Action.objects.select_related('reoccurance').filter(complete=False)

问题仍然存在，我为模板循环中的每个action.tags.all"点击了数据库.是否可以在 django-taggit 上使用某种预取?

The problem still remains that I hit the database for every "action.tags.all" in the template loop. Is it possible to use some sort of prefetch on django-taggit?

推荐答案

可以使用 prefetch_related 来检索标签，但您需要避开 'tags' 属性，因为 - 作为 jdi说 - 这是一个自定义管理器，而不是一个真正的关系.相反，您可以这样做:

It's possible to use prefetch_related to retrieve the tags, but you need to sidestep around the 'tags' property, since - as jdi says - this is a custom manager rather than a true relation. Instead, you can do:

actions = Action.objects.select_related('reoccurance').filter(complete=False).prefetch_related('tagged_items__tag')

不幸的是，您的模板代码中的 action.tags.all 不会使用预取，最终会执行自己的查询 - 因此您需要采取相当笨拙的步骤绕过标签"经理也在那里:

Unfortunately, action.tags.all in your template code will not make use of the prefetch, and will end up doing its own query - so you need to take the rather hacky step of bypassing the 'tags' manager there too:

{% for tagged_item in action.tagged_items.all %}
    <span>{{ tagged_item.tag }}</span>{% if not forloop.last %}, {% endif %}
{% endfor %}

(编者:如果您得到'QuerySet' 对象没有属性 'prefetch_related'"，则表明您使用的是低于 1.4 的 Django 版本，其中 prefetch_related 不可用.)

(Ed.: if you're getting "'QuerySet' object has no attribute 'prefetch_related'", that suggests that you're on a version of Django below 1.4, where prefetch_related isn't available.)

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