Delete field data for a single revision of an existing entity. The passed entity must have a revision id attribute.
$entity_type: The type of $entity; e.g. 'node' or 'user'.
$entity: The entity with fields to save.
function field_attach_delete_revision($entity_type, $entity) {
_field_invoke('delete_revision', $entity_type, $entity);
list($id, $vid, $bundle) = entity_extract_ids($entity_type, $entity);
// Collect the storage backends used by the fields in the entities.
$storages = array();
foreach (field_info_instances($entity_type, $bundle) as $instance) {
$field = field_info_field_by_id($instance['field_id']);
$field_id = $field['id'];
$storages[$field['storage']['type']][$field_id] = $field_id;
}
// Field storage backends delete their data.
foreach ($storages as $storage => $fields) {
$storage_info = field_info_storage_types($storage);
module_invoke($storage_info['module'], 'field_storage_delete_revision', $entity_type, $entity, $fields);
}
// Let other modules act on deleting the revision.
module_invoke_all('field_attach_delete_revision', $entity_type, $entity);
}