AsMultiMap with selector

It looks to me like AsMultiMap uses some component as a key to reference entities. Is there some way that I can specify a key selector instead of using an entire component?

For example, lets say I have some component:

struct TransformComponent {
  Vector3 Position;
  Quaternion Rotation;
  Entity Parent;
}

If I wanted to get the parent of some component, that’s easy to do by following Parent. But if I wanted to find all children of an entity, how could I construct a query to efficiently perform that lookup? Is this even a viable approach?

[Doraku]

You can provide AsMultiMap with a IEqualityComparer to change what the multimap use for equality. You can also make your component implements IEquatable<T> to change the default equality comparer used by the multimap. With both solution you can only take into account the Entity Parent and the multimap would effectively give you all children for a given entity.

[Doraku]

Hum I guess it should be possible but we would need a new type EntityMap<TComponent, TKey> with a Func<TComponent, TKey> selector like in your example (the key should still be part of a component obviously). It would be cool to also allow the key to be produced from multiple components. I will try to play around with this idea.