Java GAE: Can I filter by comparing the properties of entities?

Question

I need to query entities having equal properties, for example:

class Relation {
  Integer a;
  Integer b;
}

Query q = pm.newQuery(Relation.class);
q.setFilter("a == b");

The above query returns empty list and I haven't found any way to accomplish this task.

Answer 1

Looking into the documentation for datastore queries, it states clearly that :

The property value must be supplied by the application; it cannot refer to or be calculated in terms of other properties.

So what you're trying to do is impossible.

One thing you COULD do is this :

1- add a third property to your entity (say a boolean called "equals").

2- Whenever you update your entity, you check if a == b.

3- You update "equals" according to the check made in 2.

4- To query, you look where equals == true.