Just a quick snippet!

Dropping this in a custom module will allow you to easily index values from the Flag module's {flag_count} table in a Search API Solr index.

/**
* Get the flag count for a given node.
*/
function mymodule_get_count($entity, $options, $name, $entity_type, &$info) {
// Requiring type node since we're relying on $entity->nid,
// but this could be used for user objects too.
if ($entity_type == 'node') {
$query = db_select('flag_counts' ,'fc');
$query->fields('fc', array('count'));
$query->condition('fc.fid', $info['data']['flag']->fid);
$query->condition('fc.content_type', 'node');
$query->condition('fc.content_id', $entity->nid);
$count = $query->execute()->fetchColumn();
}
return !empty($count) ? $count : 0;
}
 
/**
* Implements hook_entity_property_info_alter().
*/
function mymodule_entity_property_info_alter(&$info) {
if (isset($info['node']['bundles'])) {
// For each content type.
foreach ($info['node']['bundles'] as $bundle_type => $bundle) {
// Find all applicable flags for this content type.
$flags = flag_get_flags('node', $bundle_type);
// For each applicable flag.
foreach ($flags as $fid => $flag) {
$info['node']['bundles'][$bundle_type]['properties']['flag_' . $flag->name . '_count'] = array(
'label' => t('@title Flag Count', array('@title' => $flag->title)),
'description' => t('The total number of @title flags for this node.', array('@title' => $flag->title)),
'type' => 'integer',
'getter callback' => 'mymodule_get_count',
'computed' => TRUE,
'data' => array('flag' => $flag),
);
}
}
}
}

After placing this in a custom module enabling, just go to the 'fields' tab on the desired Search API Solr index and select the 'flag_type Flag Count' field with type integer.

7.x,
drupal, search api, flag, solr, search, snippet