How to target first or last item in a nav children loop

Question

I'm trying to use this code:

{% nav category in categories %}

   {{ category.title }}

   {% children %}

   {% if not loop.last %} BLAH {% endif %}

{% endnav %}

It appears that the {% if not loop.last %} conditional doesn't work inside of a {% nav %} tag.

How can I target/style the first and last items in each level of a nav structure?

Answer 1

Update Sorry, I originally thought you were building nav and just showing simplified code. If you just want to list the categories then this should work:

{% nav category in categories %}
    {{- category.title -}}
    {% if not loop.last %}, {% endif %}
    {% children %}
{% endnav %}

If your trying to build a nav system, then this should work. In either case just place your 'loop' before the children tag. See this answer for another example.

<ul id="nav">
    {% nav category in categories %}

        {% if loop.first %}
            <li class="first">
        {% elseif loop.last %}
            <li class="last">
        {% else %}
            <li>
        {% endif %}

            <a href="{{ category.url }}">{{ category.title }}</a>
            {% ifchildren %}
                <ul>
                    {% children %}
                </ul>
            {% endifchildren %}
        </li>
    {% endnav %}
</ul>

Answer 2

I ended up solving this with a recursive Twig macro:

{% macro catObj(category) %}
    ... blah ...
    {% if category.children | length %}
        {% for child in category.children %}
            {{ _self.catObj( child ) }}
            {% if not loop.last %},{% endif %}
        {% endfor %}
    {% endif %}
{% endmacro %}

{% for category in categories %}
    {{ _self.catObj( category ) }}
    {% if not loop.last %},{% endif %}
{% endfor %}

I appreciate Douglas and others pointing out that the loop variable is available in the {% nav %} tag before the {% children %} tag, but in this case it seemed like less work to use a macro than to jump through code-rearranging/variable-hoisting hoops.