How use rendered variables in a foreach Symfony 2 ~ twig - php

Question

its not the first time I met this issue and I cannot fix it!
Actually, I'm rendering a template with a controller who give to the rendered page many variables. One of them, called $categories, in it, There are many of Category objectes so, one of them its a Collection what references to another Category.
The point is, I'm try to do this code, but obviusly I get an error because im trying to print as a String a Collection
{% for category in categories %}
<tr>
<td>{{ category.name }}</td>
<td>{{ category.description }}</td>
<td>{{ category.isPublic }}</td>
<td>{{ category.parentCategory }}</td>
<td>{{ category.childrens }}</td>
<td>
<i class="fa fa-pencil-square-o" aria-hidden="true"></i>
<i class="fa fa-times" aria-hidden="true"></i>
</td>
</tr>
{% endfor %}
So, I decided to try something like:
{% for category in categories %}
<tr>
<td>{{ category.name }}</td>
<td>{{ category.description }}</td>
<td>{{ category.isPublic }}</td>
<td>{{ category.parentCategory }}</td>
<td>
{% for children in {{ category.childrens }} %}
children.name
{% endfor %}
</td>
<td>
<i class="fa fa-pencil-square-o" aria-hidden="true"></i>
<i class="fa fa-times" aria-hidden="true"></i>
</td>
</tr>
PROBLEM:
I don't know how use a rendered variable in a foreach, im gettin this error:
A hash key must be a quoted string, a number, a name, or an expression enclosed in parentheses (unexpected token "punctuation" of value "{" in AppBundle:admin:category/listCategory.html.twig at line 55.

Answer 1

{{ }} or {% %} or {# #} are Twig's open and close tags. Similair to <?php ?> in PHP code. Once you use an open tag, the text is parsed by Twig until the close tag is found (this is also how things are done in PHP, the only difference is that Twig has a different tag to echo stuff).
Once open, you don't have to reopen it again. You don't want to dump category.childrens, you want to use it in the for loop. So instead of doing: {% for children in {{ category.childrens }} %}, use {% for children in category.childrens %}.
(you can compare this to PHP, doing
<?php foreach (<?php echo $category->childrens ?> as $children) { ?>
doesn't make much sense).

Answer 2

The error probably comes from this line:
{% for children in {{ category.childrens }} %}
This is not a valid syntax, the {{ }} can't be used inside another Twig tag.
The following code should work:
{% for children in category.childrens %}

Answer 3

To be honest I have 0 experience with the twig templating langauge, so I may be wrong here, but my experience with other languages tells me the following code:
{% for children in {{ category.childrens }} %}
children.name
{% endfor %}
Should probably look like this:
{% for children in category.childrens %}
{{ children.name }}
{% endfor %}