Drupal 8: Adding new fields in the database (install / uninstall)

drupal_8_adding_new_fields_in_database.txt

// Context: In a custom module, within the .install file
// Doing tasks on the installation. 
// Looking for a field and create if not exist.
// We'll delete this field in the uninstallation process.
// We're using a pair of basic Drupal hooks: install and uninstall. 

/**
 * Implements hook_install().
 */
function your_custom_module_install() {

  // Get the connection and the database schema.
  $connection = \Drupal::service('database');
  $schema = $connection->schema();

  // Test if the table 'Comment' exist or not.
  if ($schema->tableExists('comment')){
  
    // Test if the field 'extra' exist or not.
    if(!($schema->fieldExists('comment','extra'))) {

      // Build the field definition.
      $field_definition = [
        'description' => 'Newly created Extra Field.',
        'type' => 'int',
        'not null' => TRUE,
        'default' => 1,
      ];

      // Adding the new field if the field not exist.
      $schema->addField('comment', 'extra', $field_definition);

      // Set a new message.
      \Drupal::logger('your_custom_module')->notice('Created field Extra');
    }
  }
}

/**
 * Implements hook_uninstall().
 */
function your_custom_module_uninstall() {

  // Get the connection and the database schema.
  $connection = \Drupal::service('database');
  $schema = $connection->schema();

  // Test if the table 'Comment' exist or not.
  if ($schema->tableExists('comment')) {
  
    // Test if the field 'extra' exist or not.
    if ($schema->fieldExists('comment', 'extra')) {
    
    // If exist then delete the field. 
      $schema->dropField('comment', 'extra');
    // Set a new message.
    \Drupal::logger('your_custom_module')->notice('Field Extra deleted.');
    }
  }
}