Entity Relationships

[NeusFear]

I love the simplicity of the library, and plan to replace my own slow ECS implementation in my custom game engine with this, but a feature that I find to be missing is entity relationships. An example of how this could work is:

entity1.addRelationship(entity2, "hates");
entity1.addRelationship(entity3, "loves");

// then in a system you could do
Runnable system = () -> {
    //finds entities that entity1 hates
    hello.findEntitiesWithRelationship(entity1, "hates")
            //filter those entities to ones with position and velocity components
            .withAlso(Position.class, Velocity.class)
            // stream the results
            .stream().forEach(result -> {
                ...
            });
};

The benefit of such a system in a game are limitless, but here is a couple examples:

1. A group of players attack a enemy, we can add the "underAttacckBy" relationship to the enemy for each of the attacking players, then a system can loop through those players to attack the closest one, or the one with the least health, etc.
2. A player uses a fireball spell in the middle of a town center, you can get all entities in a radius around the player and put a "fears" relationship to the player to each of the surrounding NCPs and they will remember that they saw that player do something scary a while ago.

I am sure that there are more use cases, and that there is a better way to handle system results to keep your query optimizations working, but wanted your thoughts on such a feature.