Ambiguous key fields in queries

If a column name in the source key passed to the map key appears in more than one table of the source query, you will get an error along the lines of:

Integrity constraint violation: 1052 Column 'id' in on clause is ambiguous

The alias for the table containing the column instance you want to use needs to be passed to the map class constructor:

    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'bid' => array(
          'type' => 'int',
          'not null' => TRUE,
          'alias' => 'b',
        ),
      ),
      MigrateDestinationNode::getKeySchema()
    )