groups/modules/commons/commons_follow/commons_follow_group/commons_follow_group.module

<?php
/**
 * @file
 * Code for the Commons Follow (Groups) feature.
 */

include_once 'commons_follow_group.features.inc';

/**
 * Implements hook_commons_follow_get_nids().
 */
function commons_follow_group_commons_follow_get_nids($account, $options) {
  // Get all group content from a followed group.
  $gids = og_get_entity_groups('user', $account);
  if (empty($gids['node'])) {
    return;
  }

  // Get subscribe flag IDs.
  $flag_ids = commons_follow_get_subscription_flags_ids('node');

  $query = db_select('flagging', 'f');
  if (!empty($options['range'])) {
    $query->range(0, $options['range']);
  }

  $result = $query
    ->condition('fid', $flag_ids, 'IN')
    ->condition('uid', $account->uid, '=')
    ->condition('entity_type', 'node', '=')
    ->condition('entity_id', $gids['node'], 'IN')
    ->fields('f',array('entity_id'))
    ->execute()
    ->fetchAll();

  if (empty($result)) {
    return array();
  }

  $nids = array();
  foreach($result as $row) {
    $nids[] = $row->entity_id;
  }

  // Get the groups' content IDs.
  $query = new EntityFieldQuery();
  $result = $query
    ->entityCondition('entity_type', 'og_membership')
    ->propertyCondition('group_type', 'node')
    ->propertyCondition('gid', $nids, 'IN')
    ->propertyCondition('entity_type', 'node', '=')
    ->execute();

  if (empty($result['og_membership'])) {
    return array();
  }

  // Keep array structure.
  $nids = array();
  $og_memberships = og_membership_load_multiple(array_keys($result['og_membership']));
  foreach ($og_memberships as $og_membership) {
    $nid = $og_membership->etid;
    $nids[$nid] = $nid;
  }

  return $nids;
}

/**
* Implements hook_node_view_alter().
*/
function commons_follow_group_node_view_alter(&$build) {
  // Prevent users from following group nodes under the
  // commons_follow_node flag.
  // Instead, they should be followed using the commons_follow_group flag.
  if (og_is_group_type('node', $build['#node']->type)) {
    if (!empty($build['links']['flag']['#links']['flag-commons_follow_node'])) {
      unset($build['links']['flag']['#links']['flag-commons_follow_node']);
    }
  }
}

/**
* Implements hook_commons_follow_get_message_ids().
*/
function commons_follow_group_commons_follow_get_message_ids(&$followed_mids, $followed_content = array()) {
  // Generate a list of message IDs where the target nodes are followed by
  // the current user.
  $result = db_query("SELECT m.mid AS mid FROM {message} m INNER JOIN {field_data_field_target_nodes} tn ON m.mid=tn.entity_id INNER JOIN {og_membership} om ON tn.field_target_nodes_target_id=om.etid WHERE om.entity_type = 'node' AND om.gid IN (:gids)", array(':gids' => array_values($followed_content['commons_follow_group'])));
  foreach ($result as $this_result) {
    $followed_mids[] = $this_result->mid;
  }
}

/**
 * Implements hook_node_insert().
 */
function commons_follow_group_node_insert($node) {
  if ($node->type != 'group') {
    return;
  }
  // The owner of the content automatically follows the content
  $account = user_load($node->uid);
  if ($flag = flag_get_flag('commons_follow_group')) {
    $flag->flag('flag', $node->nid, $account);
  }
}


/**
 * Implements hook_og_membership_insert().
 * Invoked from hook_entity_insert().
 */
function commons_follow_group_og_membership_insert(OgMembership $og_membership) {
  commons_follow_group_sync_flag_to_group_membership($og_membership);
}

/**
 * Implements hook_og_membership_update().
 * Invoked from hook_entity_update().
 */
function commons_follow_group_og_membership_update(OgMembership $og_membership) {
  commons_follow_group_sync_flag_to_group_membership($og_membership);
}

/**
 * Implements hook_og_membership_delete().
 * Invoked from hook_entity_delete().
 */
function commons_follow_group_og_membership_delete(OgMembership $og_membership) {
  // The user should unfollow the group when her membership is deleted.
  if ($og_membership->entity_type == 'user' && $og_membership->field_name == 'og_user_node') {
    $account = user_load($og_membership->etid);
    flag('unflag', 'commons_follow_group', $og_membership->gid, $account);
  }
}

// function flag($action, $flag_name, $entity_id, $account = NULL) {

function commons_follow_group_sync_flag_to_group_membership($og_membership) {
  if ($og_membership->entity_type == 'user' && $og_membership->field_name == 'og_user_node') {
    $account = user_load($og_membership->etid);
    // The membership is approved. The user should follow the group.
    if ($og_membership->state == 1) {
      flag('flag', 'commons_follow_group', $og_membership->gid, $account);
    }
    // The membership is not approved. The user should not follow the group.
    else {
      flag('unflag', 'commons_follow_group', $og_membership->gid, $account);
    }
  }
}