How to add a property to an existing field in Drupal 8/9

README.md

How to add a property to an existing Field Type in Drupal 8/9

• Modify the schema definition of your custom field type to add a new column.
• Adjust the AddFieldProperty class to fit your needs.
• Inside a hook_update_N() function run the execute function.

AddFieldProperty.php

<?php

namespace Drupal\my_module;

use Drupal\Core\Entity\Sql\SqlContentEntityStorage;
use Drupal\Core\Field\BaseFieldDefinition;
use Drupal\Core\Field\FieldStorageDefinitionInterface;

/**
 * Adds a new property to an existing field type.
 *
 * @see: Drupal\my_module\Plugin\FieldType\CustomFieldType.php
 */
class AddFieldProperty {

  protected $entity_type_id = 'entity';

  protected $field_name = 'field';

  protected $property = 'property';

  /**
   * Builds the response.
   */
  public function execute() {
    $manager = \Drupal::entityDefinitionUpdateManager();

    $new_field_definition = BaseFieldDefinition::create('custom_field_type')
      ->setLabel(t('Label'))
      ->setDescription(t('Description'))
      ->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
      ->setDisplayOptions('view', [
        'label' => 'above',
        'type' => 'custom_field_type_formatter',
      ])
      ->setDisplayOptions('form', [
        'label' => 'above',
        'type' => 'custom_field_type_widget',
      ])
      ->setDisplayConfigurable('view', TRUE)
      ->setDisplayConfigurable('form', TRUE);

    // Overrides the current field definition.
    $manager->installFieldStorageDefinition($this->field_name, $this->entity_type_id, $this->entity_type_id, $new_field_definition);

    // Add the column for the new property in each of the field tables.
    $field_map = \Drupal::service('entity_field.manager')->getFieldMapByFieldType('custom_field_type');
    foreach ($field_map as $entity_type_id => $fields) {
      foreach (array_keys($fields) as $field_name) {
        $field_storage_definition = $manager->getFieldStorageDefinition($field_name, $entity_type_id);
        $storage = \Drupal::entityTypeManager()->getStorage($entity_type_id);

        if ($storage instanceof SqlContentEntityStorage) {
          $table_mapping = $storage->getTableMapping([
            $field_name => $field_storage_definition,
          ]);
          $table_names = $table_mapping->getDedicatedTableNames();
          $columns = $table_mapping->getColumnNames($field_name);

          foreach ($table_names as $table_name) {
            $field_schema = $field_storage_definition->getSchema();
            $schema = \Drupal::database()->schema();
            $field_exists = isset($columns[$this->property]) && $schema->fieldExists($table_name, $columns[$this->property]);
            $table_exists = $schema->tableExists($table_name);

            if (!$field_exists && $table_exists) {
              $schema->addField($table_name, $columns[$this->property], $field_schema['columns'][$this->property]);
            }
          }
        }
        $manager->updateFieldStorageDefinition($field_storage_definition);
      }
    }

    $database = \Drupal::database();
    $manager = \Drupal::entityDefinitionUpdateManager();
    $field_storage_definition = $manager->getFieldStorageDefinition($this->field_name, $this->entity_type_id);
    $entity_storage = \Drupal::entityTypeManager()->getStorage($this->entity_type_id);
    $table_mapping = $entity_storage->getTableMapping([
      $this->field_name => $field_storage_definition,
    ]);

    $table_names = $table_mapping->getDedicatedTableNames();

    foreach ($table_names as $table_name) {
      // Store the existing values.
      $table_values = $database->select($table_name)
        ->fields($table_name, [
          'bundle',
          'entity_id',
          'revision_id',
          'delta',
        ])
        ->execute()
        ->fetchAll();

      foreach ($table_values as $row) {
        $property_value = 'SOMETHING HERE';
        // Update existing data.
        $result = $database->update($table_name)
          ->fields([
            $this->field_name . '_' . $this->property => $property_value,
          ])
          ->condition('deleted', $row['deleted'])
          ->condition('entity_id', $row['entity_id'])
          ->condition('revision_id', $row['revision_id'])
          ->condition('langcode', $row['langcode'])
          ->condition('delta', $row['delta'])
          ->execute();
      }
    }

  }

}

CustomFieldType.php

<?php

namespace Drupal\my_module\Plugin\Field\FieldType;

use Drupal\Core\Field\FieldItemBase;
use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\Core\TypedData\DataDefinition;

/**
 * @FieldType(
 *   id = "custom_field_type",
 *   label = @Translation("Custom Field Type"),
 *   description = @Translation("Custom Field Type Description"),
 *   category = @Translation("Category"),
 *   default_widget = "custom_field_type_widget",
 *   default_formatter = "custom_field_type_formatter",
 * )
 */
class CustomFieldType extends FieldItemBase {

  /**
   * {@inheritDoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => 'varchar',
          'length' => 2048,
        ],
        
        # NEW PROPERTY
        'property' => [
          'type' => 'varchar',
          'length' => 255,
        ],
      ],
    ];
  }

  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    return [
      'value' => DataDefinition::create('string')->setLabel(t('Value')),
      'property' => DataDefinition::create('string')->setLabel(t('Property')),
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $value1 = $this->get('value')->getValue();
    $value2 = $this->get('property')->getValue();
    return empty($value1) || empty($value2);
  }

}