"property stemming from field" in hook_entity_property_info

Hi,

In the Entity API page, for the hook_entity_property_info it says :

field: (optional) A boolean indicating whether a property is stemming from a field.

http://www.drupalcontrib.org/api/drupal/contributions!entity!entity.api....

...but nothing more, and i was not able to find any doc or example about defining an entity property from a field.

How to declare an entity property from a field (as entities are fieldable) ?

For example, working with the group module, i can add a property the "dirty way"

With the dirty way : i create another column 'field_clean_id' in the group table, declare it with hook_schema and property with group_entity_property_info(). Actually i'm"hacking" the group module which is not great.

function group_entity_property_info() {
....
$group['field_clean_id'] = array(
    'label' => t('Identifier'),
    'description' => t('The identifier of the group.'),
    'type' => 'text',
    'setter callback' => 'entity_property_verbatim_set',
    'schema_field' => 'field_clean_id',
  );

With the clean way : i create a simple text field ('field_clean_id' in a custom module) attached to the "community" bundle from group. All is OK but how to define this field value as a property ?

Thanks !

P.S : here is how i created the fields for the group entity/community bundle, all is OK :

  function ecv_group_install() {

      // Create all the fields we are adding to our entity type.
      // http://api.drupal.org/api/function/field_create_field/7
      foreach (_mymodule_field_data() as $field) {
        field_create_field($field);
      }

      // Create all the instances for our fields.
      // http://api.drupal.org/api/function/field_create_instance/7
      foreach (_mymodule_instance_data() as $instance) {
        field_create_instance($instance);
      }
    }

    // Create the array of information about the fields we want to create.
    function _ecv_group_field_data() {
      $fields = array();

	  
	  fields['field_id_clean'] = array(
  'translatable' => '0',
  'entity_types' => array(),
  'settings' => array(
    'max_length' => '255',
  ),
  'storage' => array(
    'type' => 'field_sql_storage',
    'settings' => array(),
    'module' => 'field_sql_storage',
    'active' => '1',
    'details' => array(
      'sql' => array(
        'FIELD_LOAD_CURRENT' => array(
          'field_data_field_id_clean' => array(
            'value' => 'field_id_clean_value',
            'format' => 'field_id_clean_format',
          ),
        ),
        'FIELD_LOAD_REVISION' => array(
          'field_revision_field_id_clean' => array(
            'value' => 'field_id_clean_value',
            'format' => 'field_id_clean_format',
          ),
        ),
      ),
    ),
  ),
  'foreign keys' => array(
    'format' => array(
      'table' => 'filter_format',
      'columns' => array(
        'format' => 'format',
      ),
    ),
  ),
  'indexes' => array(
    'format' => array(
      'format',
    ),
  ),
  'field_name' => 'field_id_clean',
  'type' => 'text',
  'module' => 'text',
  'active' => '1',
  'locked' => '0',
  'cardinality' => '1',
  'deleted' => '0',
  'columns' => array(
    'value' => array(
      'type' => 'varchar',
      'length' => '255',
      'not null' => FALSE,
    ),
    'format' => array(
      'type' => 'varchar',
      'length' => 255,
      'not null' => FALSE,
    ),
  ),
  'bundles' => array(
    'group' => array(
      'community',
    ),
  ),
);
      return $fields;
      }

    // Create the array of information about the instances we want to create.
    function _ecv_group_instance_data() {
      $instances = array();

	  
	  $instances['field_id_clean'] = array(
  'label' => 'ID clean',
  'widget' => array(
    'weight' => '1',
    'type' => 'text_textfield',
    'module' => 'text',
    'active' => 1,
    'settings' => array(
      'size' => '60',
    ),
  ),
  'settings' => array(
    'text_processing' => '0',
    'user_register_form' => FALSE,
  ),
  'display' => array(
    'default' => array(
      'label' => 'above',
      'type' => 'text_default',
      'settings' => array(),
      'module' => 'text',
      'weight' => 0,
    ),
  ),
  'required' => 0,
  'description' => "L' identifiant de la collectivité qui sera synchronisé avec LDAP pour utilisation dans la GED.",
  'default_value' => NULL,
  'field_name' => 'field_id_clean',
  'entity_type' => 'group',
  'bundle' => 'community',
  'deleted' => '0',
);
	  
      return $instances;
    }