如何反转 Django 模板中的 for 循环，然后对结果进行切片 [英] How to reverse a for loop in a Django template and then slice the result

问题描述

在 Django 模板中，我遍历一组照片，并一张一张地显示它们.具体来说，现在我只有一组照片，包含 6 个对象.我像这样显示这 6 个对象:

In a Django template, I'm iterating over a set of photos, and displaying them one by one. Specifically, right now I just have one photo set, containing 6 objects. I display these 6 objects like so:

{% for pic in picstream.photo_set.all reversed %}
    <img src="{{ pic.image_file.url }}"></img>
{% endfor %}

在语句中添加 reversed 给了我所需顺序的 6 个对象(即最新的 id 首先).

Adding reversed to the statement gives me the 6 objects in the desired ordering (i.e. the latest ids first).

接下来，我想显示 photo_set 中不超过 4 个的对象.我将 |slice:":4" 添加到 picstream.photo_set.all 以实现此目的.问题是，它从我想要的排序中切断了前两个对象

Next, I want to display not more than 4 objects from the photo_set. I added |slice:":4" to picstream.photo_set.all to achieve this. Problem is, it's cutting off the first two objects from my desired oredering.

似乎应该有一种方法可以将列表先反转，然后稍后切片?需要一种简单的方法来做到这一点，而不会影响性能.

It seems there ought to have been a way to reverse the list first, and slice later? Need a simple way to do this, without performance compromises.

推荐答案

代替使用 reversed 参数作为 for 模板标签，您可以使用 查询集本身的反向方法:

Instead of using the reversed argument for the for template tag, you can use the reverse method of the queryset itself:

{% for pic in picstream.photo_set.all.reverse|slice:":4" %}
    <img src="{{ pic.image_file.url }}"></img>
{% endfor %}

如果您正在评估代码中其他地方的原始(非反向)查询集，那么这将导致第二个查询访问数据库.如果是这种情况，那么最好将逻辑移到视图代码本身或模板标记中.

If you are evaluating the original (non-reversed) queryset somewhere else in your code then this will result in a second query hitting the database. If this is the case then you are better off moving the logic into your view code itself or into a template tag.

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