CiviCRM Community Forums (archive)

I'm trying to use API chaining but not figuring out the sibling syntax.  The example https://github.com/civicrm/civicrm-core/blob/master/api/v3/examples/Contact/APIChainedArrayValuesFromSiblingFunction.php shows this but it's not clear (at least to me!) how to specify which value from the first call is used in the second call.

Specifically, I'm looking for households with their Head of Household relationship(s) and the related contact records.

$result = civicrm_api3('Contact', 'get', array(
  'sequential' => 1,
  'contact_type' => "Household",
  'api.Relationship.get' => array('contact_id_b' => "\$value.id", 'relationship_type_id' => "7"),
  'api.Contact.get' => array('id' => "XXX"),
));
What should XXX be to specify that it should use 'contact_id_a' from the previous api.Relationship.get call?  I tried "$value.api.Relationship.get.contact_id_a" but that returns many contacts (effectively 'id'=>"")