fwiw this was the answer:

@mrg why not:

(def db (ds/create-conn {:supervisors {:db/cardinality :db.cardinality/many}}))

(ds/transact db [{:name "max" :supervisors [1 2 3]}
                 {:name "dilbert" :supervisors [1 2]}])

(defn supervised-by [supervisors]
  (ds/q '[:find [?e ...]
          :in $ [?supervisor ...]
          :where
          [?e :supervisors ?supervisor]]
    @db supervisors))

(supervised-by [3])
=> [1]
(supervised-by [2 3])
=> [1 2]
(supervised-by [1])
=> [1 2]
(supervised-by [2])
=> [1 2]

(ds/q '[:find [?e ...]
        :in $ ?supervisor
        :where [?e :supervisors ?supervisor]]
  @db 3)
=> [1]

oooh, that's how cardinality works?

Thank you @misha, this is great!

@mrg you'd probably need to have

{:supervisors {:db/cardinality :db.cardinality/many :db/valueType :db.type/ref}}

omitted it for example brevity.

Thanks!

I haven't really come across datascript schemas yet. Setting the value type to ref would mean that this is a reference to another entity, right?

I think you need to invest few hours in these: http://www.datomic.com/training.html

yes

I think so too. Thanks for the pointer.

and look up stuff here too: http://docs.datomic.com/schema.html

That's what I'm reading right now 🙂

it might not behave exactly as in datomic, but it'll get you 95% (or even 100% for many use cases) there

Probably close enough.

It's a very big topic; too big to dive in without a project to guide the reading, but then you come across things like my problem that you don't understand without having seen the right approach at some points

And then you're at an impasse unless someone points you in the right direction. Thanks again

I know what to read up on next now 🙂

I'd start with videos

the stu and tim ewalds videos were a good place to start for me