I'm trying to override a form widget in Twig following this documentation:
<http://symfony.com/doc/current/cookbook/form/form_customization.html#method-1-inside-the-same-template-as-the-form>

But I'm a little bit lost.
Here is the **select** field I want to breakdown:

```
{{ form_widget(edit_form.activities) }}

```

Here is the overriding process taken in the doc:

```
    {% form_theme edit_form _self %}

    {%- block choice_widget_options -%}
        {% for group_label, choice in options %}
            {%- if choice is iterable -%}
                <optgroup label="{{ choice_translation_domain is same as(false) ? group_label : group_label|trans({}, choice_translation_domain) }}">
                    {% set options = choice %}
                    {{- block('choice_widget_options') -}}
                </optgroup>
            {%- else -%}
                {% set attr = choice.attr %}
                <option value="{{ choice.value }}" {{ block('attributes') }}{% if choice is selectedchoice(value) %} selected="selected"{% endif %}>{{ choice_translation_domain is same as(false) ? choice.label : choice.label|trans({}, choice_translation_domain) }}</option>
            {%- endif -%}
        {% endfor %}
    {%- endblock choice_widget_options -%}

```

The purpose is to be able to select multiple field in the select box with an array of ids posted through my controller.

You guys have any idea how to do it?