public function DatabaseConnection_mysql::nextId in Drupal 7

Retrieves an unique id from a given sequence.

Use this function if for some reason you can't use a serial field. For example, MySQL has no ways of reading of the current value of a sequence and PostgreSQL can not advance the sequence to be larger than a given value. Or sometimes you just need a unique integer.

Parameters

$existing_id: After a database import, it might be that the sequences table is behind, so by passing in the maximum existing id, it can be assured that we never issue the same id.

Return value

An integer number larger than any number returned by earlier calls and also larger than the $existing_id if one was passed in.

Overrides DatabaseConnection::nextId

File

includes/database/mysql/database.inc, line 483

Database interface code for MySQL database servers.

Class

DatabaseConnection_mysql

Code

public function nextId($existing_id = 0) {
  $new_id = $this
    ->query('INSERT INTO {sequences} () VALUES ()', array(), array(
    'return' => Database::RETURN_INSERT_ID,
  ));

  // This should only happen after an import or similar event.
  if ($existing_id >= $new_id) {

    // If we INSERT a value manually into the sequences table, on the next
    // INSERT, MySQL will generate a larger value. However, there is no way
    // of knowing whether this value already exists in the table. MySQL
    // provides an INSERT IGNORE which would work, but that can mask problems
    // other than duplicate keys. Instead, we use INSERT ... ON DUPLICATE KEY
    // UPDATE in such a way that the UPDATE does not do anything. This way,
    // duplicate keys do not generate errors but everything else does.
    $this
      ->query('INSERT INTO {sequences} (value) VALUES (:value) ON DUPLICATE KEY UPDATE value = value', array(
      ':value' => $existing_id,
    ));
    $new_id = $this
      ->query('INSERT INTO {sequences} () VALUES ()', array(), array(
      'return' => Database::RETURN_INSERT_ID,
    ));
  }
  $this->needsCleanup = TRUE;
  return $new_id;
}