When we created our TaskType class, we skipped the tags property as this was not a simple text or date field. The tags property is related to the Tag table with the M:N relation, so we could present it as a list of checkboxes or a select box. However, people are used to fill tags with comma-separated values, so let's try to convert our objects to comma-separated tag names when displaying a field and convert them back when saving data to the database.

To present a list of objects (tags) as comma-separated values and to convert them back to tags, we need to create a data transformer. A data transformer is a class implementing DataTransformerInterface and allows us to transform our data to various formats. As an example—a data transformer is used when the date or date/time field type is used.

Create a src/AppBundle/Form/DataTransformer/TagDataTransformer.php file as follows:

<?php

namespace AppBundle\Form\DataTransformer;

use AppBundle\Entity\Tag;
use Doctrine\ORM\EntityManager...