In this tutorial, we will see how to add field to custom schema in Drupal 8 without losing data using a hook_update_N().
Example how to use hook_update_N() :
<?php
use Drupal\Core\Database\Database;
/**
* @param $sandbox
*/
function mymodule_update_90002(&$sandbox) {
$spec = [
'type' => 'varchar',
'description' => "New Col",
'length' => 20,
'not null' => FALSE,
];
$schema = Database::getConnection()->schema();
$schema->addField('mymodule', 'newcol', $spec);
$schema->addIndex('mymodule', 'newcol',['newcol'],$spec);
return t('new col added to mymodule table.');
}
After this, running drush updb or running update.php from the admin interface should detect your hook_update_N() and it should add your new field.
