how would you query the ids of all the entities that reference a given entity, including transitively? i.e. I have entity A, which has a :db.type/ref attribute whose value points to B, and B also has a ref attr whose value points to C, and I want to go from C to A this is trivial to do if you know how many steps there are, but implementing it for the general case seems difficult to me without resorting to complicated graph traversal algos

so is there a simpler way or do I need to do it the hard way? (i.e. graph traversal custom fn)

just to make my case clearer, this is to detect when I need to reindex an entity in elasticsearch. if a subentity (a component, usually, but not always) is changed I'll need to reindex the parent entity, but the link isn't always direct

It seems like Datomic rules would work great for this. The question is whether you can have a list of attributes that you can lookup in these rules or whether you should consider any attribute of type ref.

Absolutely not tested but I would try something in this spirit:

[(parent ?p ?e)
 [?p ?attr ?e]
 [?attr :db/type :db.type/ref]]
[(ancestor ?a ?e)
 (parent ?a ?e)]
[(ancestor ?a ?e)
 (parent ?p ?e)
 (ancestor ?a ?p)]

You might not even need the clause on the type of the attribute.

I'm absolutely blown away, I never used rules before, but this works and I'm very grateful for your help

No problem. Usually if you have a recursive problem, rules help.

@joelsanchez as noted it will be much faster if you specify the exact attribute you need

[?p ?attr ?e] <- not binding the attribute causes a much larger scan

?e should be bound to the child entity so the scan should not be much bigger, unless there are a lot of attributes on each entity they don't care about.

You should of course call parent and ancestor with a bound ?e to find ?p. Not try to retrieve the whole database 🙂

nah, they are small entities, and they don't have that many ref attributes. since the child entities are usually components, they aren't referenced by more than one entity, and the depth is always lower than 3