ENH: Add lock(bean) method as convenience for DB pessimistic locking

[rbygrave]

This will effective load or refresh the bean and obtain a database row lock using select for update

   try (Transaction transaction = DB.beginTransaction()) {

     // fetch something that relates to what we want to lock
      Order order = DB.find(Order.class, o0.getId());

      // we have a reference bean (only has id) but could be partial or fully loaded
      Customer customer = order.getCustomer();

      // load and obtain database row lock
      // load the customer bean using select for update
      // bean is now loaded and database row lock held until commit
      database.lock(customer); 
 
     ...
      transaction.commit();
    }

More fully explained with asserts

    try (Transaction transaction = DB.beginTransaction()) {

      LoggedSql.start();
      Order order = DB.find(Order.class, o0.getId());
      assert order != null;

      Customer customer = order.getCustomer();
      assertEquals(customerId, customer.getId());
      assertTrue(DB.beanState(customer).isReference());

      DB.lock(customer); // load the customer bean using select for update
      // bean is now loaded and database row lock held until commit
      assertFalse(DB.beanState(customer).isReference());

      assertThat(customer.getName()).isNotNull();

      List<String> sql = LoggedSql.stop();
      assertThat(sql).hasSize(2);
      assertThat(sql.get(0)).contains("from o_order");
      assertThat(sql.get(1)).contains("from o_customer t0 where t0.id = ?   for update");

      transaction.commit();
    }