Relations

Relations covers declaring and querying them. Underneath, an edge isn’t a separate kind of storage, it’s an ordinary component wearing a different shape.

An edge is one component, however many targets

Adding a relation edge doesn’t add a new archetype-level entry per edge. Every entity’s outgoing edges for one relation type live in a single component, RelationLinks<T>, holding all of that entity’s targets together. An entity targeting five others at once and one targeting a single entity both occupy the exact same archetype bit, adding a tenth target changes the component’s contents, not the entity’s shape. That’s also the whole story behind WithRelation<T>() being sugar for With<RelationLinks<T>>(), it’s filtering on a real component like any other.

Reverse lookups get their own storage

Finding every entity with an edge pointing at a given target isn’t something RelationLinks<T> can answer without scanning every entity that might have one. A separate component, RelationBacklinks<T>, exists to make that lookup direct instead, maintained alongside the forward edges so cascading destruction (see Parent/Child) can find every dependent entity without a linear scan.

Next

Change Tracking covers watching an entity’s relation edges change over time, not just reading their current state.