Add properties to an entity hook_schema_alter

Question

I need to implement a custom module that adds fields and properties to an entity defined in another module. I felt the best way to do this was using hook_schema_alter and hook_update_n to achieve the data structure updates. However, I do not see these new fields on my entity. Is there another step to take to attach these to the entity via code?

Answer 1

Using the third-party Entity API module, you can:

• Implement hook_schema_alter().
• Use something like db_add_field() to actually make the change to the DB in YOURMODULE_install() or updateN(). hook_schema_alter() does not actually create the field in the database. It just tells other modules at the code level that the field exists in the schema.
• Implement hook_entity_property_info_alter(). This tells the Entity API module that this field exists as a property of some entity, where it lives in the database table, and how to access it.
  • Clear the cache to have the new properties showing up.

An example property in the $info array is basically the following one:

  $properties['survey_count_to_complete'] = array(
    'label' => t('Number of Surveys to Complete'),
    'type' => 'integer',
    'description' => 'When a student logs in the system derives how many surveys they must complete. We store this value to make business logic easier.',
    // This is the schema field you made.
    'schema field' => 'survey_count_to_complete',
    'getter callback' => 'entity_property_verbatim_get',
    'setter callback' => 'entity_property_verbatim_set',
  );

Answer 2

Here is a working example which implements hook_schema_alter:

/*
 * Implements hook_schema_alter().
 */
function mymodule_schema_alter(&$schema) {
  $schema['tablename']['fields']['fieldname'] = array(
    'type' => 'varchar',
    'default' => null
  );
}

/*
 * Implements hook_install().
 */
function mymodule_install() {
  if (!db_field_exists('tablename', 'fieldname')) {
    $schema = drupal_get_schema('tablename');
    db_add_field('tablename', 'status', $schema['fields']['fieldname']);
  }
}

/*
 * Implements hook_uninstall().
 */
function mymodule_uninstall() {
  if (db_field_exists('tablename', 'fieldname')) {
    $schema = drupal_get_schema('tablename');
    db_drop_field('tablename', 'fieldname');
  }
}

Where mymodule is your module name, tablename your table name and fieldname is your field name.