Question

I have a self-joining table where each folder has a parent, and the depth of this is unlimited. One folder can have another folder as a parent, no restriction on the depth.

Today my code looks like this, and I am looking for a way of digging down as deep as it needs without hard-coding each step down, is there perhaps a way to define a twig function with a loop, that calls itself on each round in the loop?

<select id='parent' name='container'>
    <option value='none'>No parent</option>
        {% for folder in folders %}
            <option value='{{ folder.id }}'>{{ folder.name }}</option>
                {% for folder in folder.children %}
                    <option value='{{ folder.id }}'>&nbsp;&nbsp;&nbsp;{{ folder.name }}</option>    
                {% endfor %}
        {% endfor %}
</select>  

Answer 1

You need a separate file rendering options that recursively includes itself:

<select>
    <option value="none">No parent</option>
    {% include 'options.html.twig' with {'folders': folders, 'level': 0} %}
</select>

options.html.twig:

{% for folder in folders %}
    <option value="{{ folder.id }}">
        {% for i in range(0, level) %}&nbsp;{% endfor %}
        {{ folder.name }}
    </option>

    {% include 'options.html.twig' with {'folders': folder.children, 'level': level + 1} %}
{% endfor %}

I wrote this code right here, so don’t expect it to be correct, but it should be enough to give you the idea.