Adding referencing entities using hook_entity_property_info_alter I have a custom entity "Matching" with a multi value field entity reference jobs that references a node of type job.

function custom_alters_entity_property_info_alter(&amp;$info) {
  $info['node']['properties']['referencing_matching'] = array(
    'label' =&gt; t('Referencing matching'),
    'type' =&gt; "entities",
    'description' =&gt; t("Matchings referecing a job"),
    'getter callback' =&gt; 'get_referencing_matching'
  );
}

function referencing_matching($node) {
  if($node-&gt;type == "job"){
    $query = new EntityFieldQuery();
    $query-&gt;entityCondition('entity_type', 'matching')
          -&gt;fieldCondition('field_job_id', 'target_id', $node-&gt;nid, '=');
    $result = $query-&gt;execute();
    if ($result) {
      return $result;
    }
}


the "getter callback" isn't expecting an id rather than an array
It looks like you are trying to add a multi-valued entity reference property to a standard node entity. If that's the case, I think you first need to declare the property as multi-valued in hook_entity_info_alter() via the 'type' entry:

$info['node']['properties']['referencing_matching'] = array(
  'label' =&gt; t('Referencing matching'),
  'type' =&gt; 'list&lt;matching&gt;',
  'description' =&gt; t("Matchings referecing a job"),
  'getter callback' =&gt; 'get_referencing_matching'
);


See the hook_entity_property_info() notes for more on that.

Then, in the getter callback I believe you need to return a list of entity ids, something like:

if ($node-&gt;type == "job"){
  $query = new EntityFieldQuery();
  $query-&gt;entityCondition('entity_type', 'matching')
        -&gt;fieldCondition('field_job_id', 'target_id', $node-&gt;nid, '=');
  $result = $query-&gt;execute();
  if (!empty($result['matching'])) {
    return array_keys($result['matching']);
  }
}


I have not tested that specific code, but this was the pattern that I used in a similar example of my own.