I have a Twig template where I'd like to merge in an array of items as classes. So far I have this code:

{% set classes = [
  'media--type-' ~ media.bundle()|clean_class,
  not media.isPublished() ? 'media--unpublished',
  view_mode ? 'media--view-mode-' ~ view_mode|clean_class,
] %}

 {% for item in content.field_category | children(true) %}
   {% set classes = classes|merge(item) %}
 {% endfor %}

<section{{ attributes.addClass(classes) }}>


Note that {% for item in content.field_category | children(true) %} is using Twig Tweak.

In this case for example, item represents term values for each item in plain text as:


and then with <section{{ attributes.addClass(classes) }}>, I'd like the set classes as well as my custom classes to all be merged together. However, I am getting an error:

Notice: Array to string conversion in Drupal\Core\Template\AttributeArray->__toString() (line 73 of core/lib/Drupal/Core/Template/AttributeArray.php).

So this tells me, I did not merge the array of items properly but I am lost after this, not sure how to fix it. My guess is, I need to do this without a for loop somehow?


Twig's merge filter expects an array as an argument, not a string.

Updating your for loop should resolve:

{% for item in content.field_category|children(true) %}
  {# Merge classes array with an array with one item, item #}
  {% set classes = classes|merge([item]) %}
{% endfor %}
  • Maybe you could also just do {% set classes = classes|merge(content.field_category|children(true)) %}
    – sonfd
    Feb 20 at 18:18
  • Thank you this worked, I actually needed to do: {% for item in content.field_category | children(true) %} {% set classes = classes|merge([item["#plain_text"]]) %} {% endfor %} ... as that was the actual item in Xdebug. Thank you! Feb 20 at 18:24
  • As you are using Twig Tweak you could use with filter like this {%set classes = classes|with(item) %}
    – ya.teck
    Mar 13 at 17:17

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