'Returns a list of encoded ids for users the item has been shared.
Skipped if the requesting user is not the owner.'

def serialize_users_shared_with(self, item, key, user=None, **context):

    if (not self.manager.is_owner(item, user)):
        self.skip()
    share_assocs = self.manager.get_share_assocs(item)
    return [self.serialize_id(share, 'user_id') for share in share_assocs]
