//Bi-directional subscription adding
public boolean addSubscription(String from, String to){
Query q_from = new Query(ACCOUNT_KIND);
q_from.addFilter(ACCOUNT_ID_PROPERTY, FilterOperator.EQUAL, from);
q_from.setKeysOnly();
PreparedQuery pq_from = datastore.prepare(q_from);
Entity e_from = pq_from.asSingleEntity();
Query q_to = new Query(ACCOUNT_KIND);
q_to.addFilter(ACCOUNT_ID_PROPERTY, FilterOperator.EQUAL, to);
q_to.setKeysOnly();
PreparedQuery pq_to = datastore.prepare(q_to);
Entity e_to = pq_to.asSingleEntity();
if(e_from == null || e_to == null)
return false;
Key k_from = e_from.getKey();
Key k_to = e_to.getKey();