问Drupal 8迁移插件-需要解释

EN

id: book_content
label: Book content
migration_group: example
source:
  plugin: book_content
  target: db_migration
destination:
  plugin: entity:node
process:
  type:
    plugin: default_value
    default_value: article
  title: title
  uid:
    plugin: default_value
    default_value: 1
  sticky:
    plugin: default_value
    default_value: 0
  status:
    plugin: default_value
    default_value: 1
  'body/value': content
  'body/summary': excerpt
  'body/format': 
    plugin: default_value
    default_value: full_html
  # Use of the migration process plugin.
  # That will use the id map of the specified migration 
  # to retrieve the corresponding id ('source' parameter) in the destination database.
  field_description:
    plugin: migration
    source: body

<?php
/**
 * @file
 * Contains \Drupal\docapi_migrate\Plugin\migrate\source\BookContent.
 */
namespace Drupal\example_migrate\Plugin\migrate\source;
use Drupal\migrate\Plugin\migrate\source\SqlBase;
use Drupal\migrate\Row;
/**
 * Drupal 8 node from database.
 *
 * @MigrateSource(
 *   id = "book_content"
 * )
 */
class BookContent extends SqlBase {
  /**
   * {@inheritdoc}
   */
  public function query() {
    $query = $this->select('docapi_migrate', 'dm')
                  ->fields('dm', array('url', 'depth', 'body', 'meta'));
    return $query;
  }
  /**
   * {@inheritdoc}
   */
  public function fields() {
    $fields = array(
      'url' => $this->t('Content url'),
      'depth' => $this->t('Depth of the content'),
      'body' => $this->t('Full text of the content'),
      'meta' => $this->t('yaml data'),
    );
    return $fields;
  }

drush migrate-import book_content

The drush command 'migrate-import book_content' could not be found.