Fieldmanager autocomplete w/ datasource

fieldmanager.php

$datasource_speakers = new Fieldmanager_Datasource_Post( array(
  'query_args' => array( 'post_type' => array( 'speaker' ) ),
  'use_ajax'   => true
) );

$fm = new Fieldmanager_Autocomplete( __( 'Speaker' ), array(
    'name'           => 'speakers',
    'datasource'     => $datasource_speakers,
    'sortable'       => true,
    'limit'          => 0,
    'add_more_label' => __( 'Add speaker' ),
  )
);

$fm->add_meta_box( 'Speakers', array( 'page' ) );