How to use prepareKey() for the Migrate module

Question

I am trying to do write a migration class extending the Migrate module. The migration will be from an XML file so I will want to extend the XMLMigration class. Based on the Migrate modules wine.inc example, I'm going to have to something like this:

class WineProducerXMLPullMigration extends XMLMigration {
 public function __construct($arguments) {
  ....  
  $item_xpath = '/producers/producer';  // relative to document
  $item_ID_xpath = 'sourceid';          // relative to item_xpath

  $this->source = new MigrateSourceXML($items_url, $item_xpath,
                                       $item_ID_xpath, $fields);
  }
}

My issue is that my xml doesn't have any unique ID per row, and therefore I can't really pass a $item_ID_path. I read this post https://www.drupal.org/node/1444356#comment-6974360 and found that the prepareKey() function is supposed to be used for this purpose, however as far as I can tell, you don't ever call this directly, and its called after this class is finished processing. This being the case, I'm not sure what I should be passing in $item_ID_xpath and it is a required field.

Answer 1

You can set the $item_ID_xpath to something that does exist in the XML, for example 'name'

Then inside your migration class, you can use the prepareKey method to generate an ID

public function prepareKey($source_key, $row) {
  $row->glid = crc32($row->xml->name);
  $key['glid'] = $row->glid;
  return $key;
}

I am using crc32() to generate an ID from a string, but I have no clue whether that is smart or not, so use at own risk.

Now that you have an ID ('glid' in the example) you use in the mapping:

$this->map = new MigrateSQLMap($this->machineName, array(
  'glid' => array(
    'type' => 'varchar',
    'length' => '255',
    'not null' => TRUE,
    'description' => 'id generated from a string'
  ),
), MigrateDestinationNode::getKeySchema()
);

Dont forget to deregister the migration if you change this schema.