Thanks @U08JKUHA9 but the values themselves may point to other entities.

{:conversion/buy         {:db/valueType :db.type/ref
                            :db/cardinality :db.cardinality/one}
   :conversion/sell        {:db/valueType :db.type/ref
                            :db/cardinality :db.cardinality/one}
   :conversion/account     {:db/valueType :db.type/ref
                            :db/cardinality :db.cardinality/one}
   ;;----------------------------------------;;
   :arbitrage-unit/conversion  {:db/valueTpe  :db.type/ref
                                :db/unique    :db.unique/value}
   :arbitrage-unit/value       {:db/cardinality :db.cardinality/one
                                :db/doc "A arbitrage-unit's value limit"}
   :arbitrage-unit/priority    {:db/cardinality :db.cardinality/one}
   :arbitrage-unit/min-ratio   {:db/cardinality :db.cardinality/one}
   :arbitrage-unit/max-ratio   {:db/cardinality :db.cardinality/one}
   ;;----------------------------------------;;
   :equity/isin            {:db/unique    :db.unique/value}
   ;;----------------------------------------;;
   :account/name           {:db/unique    :db.unique/value}
   :account/code           {:db/unique    :db.unique/value}
   ;;----------------------------------------;;
   }

This is my schema right now, and the :conversion is the entity that is supposed to be unique for each combination of values for its field.

This is not possible right now unfortunately. You could use a transaction function to update these entities, and have the transaction function check to make sure the combined attributes are unique. But that doesn't stop another transaction from violating the desired constraint.

Thanks alot @U05100J3V, a quick question though, what differences make this impossible in datascript compared to datomic? (or is a problem on that as well)

Composite keys are also not implemented in datomic, fwiw

Thanks

Correct; You'd have to use transaction functions in Datomic as well.