How to ensure that the AnQiCMS template does not cause page rendering errors when the variable is `nil`?

When using AnQiCMS to build a website, we often encounter the situation where template variables may be empty (nilThe case where. For example, a document may not have a thumbnail set, or a custom field may not be filled in.If the template code does not properly handle these potential null values, errors may occur during page rendering, affecting user experience and even causing some website functions to be unavailable.Fortunately, AnQiCMS's template engine provides various mechanisms to help us elegantly handle these variables asnilto ensure the stability and smoothness of the page.}

AnQiCMS's template syntax is similar to the Django template engine, it uses double curly braces{{变量}}to output variable content, as well as single curly braces and percent signs{% 标签 %}To control the logical flow. Understanding these basic grammars and how to use them in conjunction with filters (filters) and tags (tags) is to avoidnilThe key that causes rendering errors due to variables.

1. Useiftag for conditional judgment

The most direct and effective method is to useifTags make conditional judgments on variables. In AnQiCMS templates,{% if 变量 %}Syntax determines if a variable is “defined”. Typically, this means the variable is notniland not an empty string""nor a number0or a boolean valuefalse.

For example, we want to display the description of the article, but some articles may not have filled in the description. If output directly{{ archive.Description }}When the description is empty, the page may display a blank screen or cause errors in some strict environments. We can handle it like this:

{% if archive.Description %}
    <p>{{ archive.Description }}</p>
{% else %}
    <p>暂无文章简介。</p>
{% endif %}

If we need to ensure that a certain image field (such asarchive.Logoorarchive.ThumbRender only if it exists<img>Then tagifIt is very practical to judge

{% if archive.Logo %}
    <img src="{{ archive.Logo }}" alt="{{ archive.Title }}" />
{% else %}
    <!-- 方案一：显示一个默认占位图 -->
    <img src="/static/images/default-logo.png" alt="默认图片" />
    <!-- 方案二：或者干脆不显示图片 -->
{% endif %}

For boolean variables such asitem.HasChildren)ifThe tag can also well judge its true or false, thereby deciding whether to render the child element:

{% if item.HasChildren %}
    <ul>
        <!-- 渲染子分类列表 -->
    </ul>
{% endif %}

By{% if not 变量 %}Sentences, we can also conveniently check whether the variable isnilor null, and provide alternative content, for example when handling 'Previous/Next' document navigation:

{% prevArchive prev %}
上一篇：
{% if not prev %}
  <span>没有了</span>
{% else %}
  <a href="{{ prev.Link }}">{{ prev.Title }}</a>
{% endif %}
{% endprevArchive %}

Second, usingdefaultanddefault_if_noneThe filter provides a default value

AnQiCMS template engine providesdefaultanddefault_if_noneA filter that provides a preset default value when a variable is empty. This is very convenient for handling empty values of variables inline.

defaultThe filter will replace the variable withnilan empty string""numbers0or a boolean valuefalseProvide a default value. For example, ifarchive.Titleis empty, we can make it display "No Title Article":

<h1>{{ archive.Title|default:"无标题文章" }}</h1>

For imagessrcProperty,defaultThe filter can also be used, to ensure that even if the image path is empty,srcProperties can also have a valid default value to prevent the browser from displaying broken image icons:

<img src="{{ archive.Thumb|default:"/static/images/placeholder.jpg" }}" alt="{{ archive.Title|default:"文章图片" }}" />

Anddefault_if_noneThe filter is more precise, it will only trigger when the variable is strictly equal to:nil(i.e., in Go language)nilProvide a default value when a variable is empty. This is very useful in scenarios where it is necessary to distinguish between 'not set' and 'set to zero/empty'.""numbers0or a boolean valuefalsebut notnil, thendefault_if_noneWill not interfere. This is very useful in scenarios where it is necessary to distinguish between 'not set' and 'set to zero/empty'.

For example, a numeric fieldarchive.PriceMay be set as0(indicating free), may also benil(indicating unknown price). If0is a valid value, whilenilthen it is necessary to default,default_if_noneMore suitable:

价格：{{ archive.Price|default_if_none:"价格待定" }} 元

Third, in the loopemptyStatement handles empty list

When we need to traverse a list (array or slice), if the list is empty, we can directly use{% for %}A loop may cause nothing to be displayed on the page, confusing the user. AnQiCMS providesforloop tags to{% empty %}solve this problem.

In{% for %}use labels inside{% empty %}When the traversed list is empty,{% empty %}and{% endfor %}the content between them will be rendered to provide a friendly prompt.

For example, when displaying an article list:

{% archiveList archives with type="list" limit="10" %}
    {% for item in archives %}
        <li><a href="{{ item.Link }}">{{ item.Title }}</a></li>
    {% empty %}
        <li>目前还没有任何文章。</li>
    {% endfor %}
{% endarchiveList %}

So, even ifarchivesThe list is empty, the page will not appear blank, but will display a prompt saying “There are no articles at the moment.”.

4. Combination usage: Build a more robust template.

In practical development, we often need to combine the above strategies to build more robust and less error-prone templates.

For example, a content block may include a title, description, and image. We want to display the entire content block if a title or description exists, and the image has a default value:

`twig {% if archive.Title or archive.Description %}

<div class="article-summary">
    <h2>{{ archive.Title|default:"最新动态"