How do I migrate Relations?

As explained on the project page, support for contributed modules should go into the contrib module itself, or into Migrate Extras.

This has not previously been requested.

I am interested in the ability to migrate individual Relations (I have built my Relation Types manually).

It seems this is being looked into by BTMash on the Relation issues queue: #1053108: Upgrade path from *reference?

I'll be working on this over the next week. I'll report something when we have something.

Cross linking: #1606404: Migrate integration & upgrade from *reference

Linked issue includes a patch.

Closing this in favor of the Relation issue #1606404: Migrate integration & upgrade from *reference.

Just an example how to migrate relations.
Note: enable relation_migrate module to load relation_migrate.destination.inc or just include it inside yours.

/**
 * @file
 * Migration for relation entities.
 */

/**
 * Class DwRelationBaseImport.
 *
 * Base class for all relations.
 * The common thing to map two source fields for endpoints,
 * and later use them in prepare method.
 */
abstract class DwRelationBaseImport extends Migration {
  /**
   * {@inheritdoc}
   */
  public function getDestinationPk() {
    return MigrateDestinationRelation::getKeySchema();
  }

  abstract public function getInstanceName();

  /**
   * {@inheritdoc}
   */
  public function fields() {
    $fields = array();

    $fields['left_entity_id']  = t('First Endpoint Entity ID');
    $fields['right_entity_id'] = t('Seconds Endpoint Entity ID');

    return $fields;
  }

  /**
   * {@inheritdoc}
   */
  public function __construct($arguments = array()) {
    parent::__construct($arguments);
    
    $this->query = $this->query();

    $this->source = new MigrateSourceSQL(
      $this->query,
      $this->fields(),
      NULL,
      array('map_joinable' => FALSE)
    );

    $this->map = $this->buildMap();

    $this->destination = new MigrateDestinationRelation(RELATION_MACHINE_NAME);

    $this->addFieldMapping('endpoints', NULL)->defaultValue(array());
    $this->addFieldMapping('left_entity_id', 'left_entity_id');
    $this->addFieldMapping('right_entity_id', 'right_entity_id');

    $this->addUnmigratedDestinations(array(
      'is_new',
      'path',
    ));
    
  }

  /**
   * Build map using class overrides.
   *
   * @return MigrateSQLMap
   *   An db-based MigrateMap.
   */
  private function buildMap() {
    return new MigrateSQLMap($this->getInstanceName(),
      $this->getSourcePK(),
      $this->getDestinationPK()
    );
  }

  /**
   * {@inheritdoc}
   */
  public function prepareRow($row) {
    if (parent::prepareRow($row) === FALSE) {
      return FALSE;
    }

    $row->right_entity_id = trim($row->right_entity_id);
    $row->left_entity_id  = trim($row->left_entity_id);

    if (empty($row->right_entity_id) || empty($row->left_entity_id)) {
      return FALSE;
    }
  }

  /**
   * {@inheritdoc}
   */
  public function prepare(stdClass $relation, stdClass $source_row) {
    $left_entity_id  = $relation->left_entity_id;
    $right_entity_id = $relation->right_entity_id;

    unset($relation->left_entity_id);
    unset($relation->right_entity_id);

    if (empty($left_entity_id) || empty($right_entity_id)) {
      $this->queueMessage(
        t('Import item failed <br>left entity id: %left,<br>right entity id: %right)', array(
          '%left' => var_export($left_entity_id, TRUE),
          '%right' => var_export($right_entity_id, TRUE),
        )),
        MigrationBase::MESSAGE_WARNING
      );

      // Ignore saving of a relation.
      $relation->rid    = FALSE;
      $relation->is_new = FALSE;

      return FALSE;
    }

    $relation->endpoints[LANGUAGE_NONE] = array(
      array(
        'entity_type' => $this->getLeftEntityType(),
        'entity_id'   => $left_entity_id,
      ),
      array(
        'entity_type' => $this->getRightEntityType(),
        'entity_id'   => $right_entity_id,
      ),
    );
  }

  /**
   * Entity type for first endpoint entity.
   */
  public function getLeftEntityType() {
    return 'user';
  }

  /**
   * Entity type for second endpoint entity.
   */
  public function getRightEntityType() {
    return 'node';
  }

}

/**
 * Class DwRelationUserAndPersonImport.
 *
 * Base class to import relations between Users and Persons.
 */
class DwRelationUserAndPersonImport extends DwRelationBaseImport {
  /**
   * {@inheritdoc}
   */
  public function getInstanceName() {
    return 'DwRelationUserAndPerson';
  }

  /**
   * {@inheritdoc}
   */
  public function getSourcePk() {
    return array(
      'person_id' => array(
        'type' => 'varchar',
        'length' => 20,
        'not null' => TRUE,
      ),
    );
  }

  /**
   * {@inheritdoc}
   */
  public function query() {
    $query = $this->sourceConnection
      ->select('cont_person', 'cp')
      ->orderBy('cp.person_id', 'ASC');

    $query->addField('cp', 'person_id');
    $query->addField('cp', 'person_id', 'left_entity_id');
    $query->addField('cp', 'person_id', 'right_entity_id');

    return $query;
  }

  /**
   * {@inheritdoc}
   */
  public function __construct($arguments = array()) {
    parent::__construct($arguments);

    $this->addFieldMapping('left_entity_id', 'left_entity_id')
      ->sourceMigration(
        'DwUserRegular',
        'DwUserAdvanced',
        'DwUserModerator'
      );

    $this->addFieldMapping('right_entity_id', 'right_entity_id')
      ->sourceMigration('DwVideoFromPerson');
  }

}

@vlad.dancer
I also have to do a migration of d7 site , relation, to another d7 site, relation. I am currently seeing that relation_migrate does not have
relation to relation migration.. only references to relation.

I opened up issue:
relation_migrate migrate from relation to relation.
https://www.drupal.org/node/2551021

Currently prepping to do it at a custom drush script level because im thinking endpoints are different entity types & bundles.. which on my end i've done as different migrations..

Hello All,

Migration classes that facilitates migration of relation now available at relation module.
Also in the tradition of migrate_d2d, an example module relation_migrate_example is also provided.

Issue at:
relation_migrate migrate from relation to relation.
https://www.drupal.org/node/2551021