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Sign upparameterized query with lookup ref returns different result to inlined lookup ref #335
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I wanted to compare to datomic but it behaves even differently: user=> (db/q '[:find ?id :where [[:id :b] :id ?id]] (db/db c))
IllegalArgumentExceptionInfo :db.error/not-a-binding-form Invalid binding form: :id datomic.error/arg (error.clj:57) |
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Thanks for reporting! What do you thinkg the correct behaviour should be? |
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I actually haven't thought of that, I guess I was expecting you to have a preference, wrt backward compatibility. Thinking about lookup refs in a recursive rule I'd expect the unification to stop, not throw. Therefore allowing a non-matching lookup ref seems reasonable. Reading datomic's docs they allow lookup refs in a parameterized query. I don't have a laptop right now, if you want to check to be compatible, feel free. |
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I checked with datomic-free and it actually throws. The model is a bit different but the point is the same: user=> (d/q '[:find ?foo :in $ ?id :where [?id :foo ?foo]] (d/db c) [:id :c])
Execution error (IllegalArgumentException) at datomic.datalog/resolve-id (datalog.clj:272).
Cannot resolve key: [:id :c]I find this inconsistent because I thought of lookup refs as shortcuts, i.e. What about a rule like It seems it's hard to model this in a way that keeps backward compatibility and datomic compatibility and consistency at the same time. If you care more about:
I'd understand if you didn't want to break userspace or if some clients expected datomic compatibility, but my vote would go for consistency, then backward compatibility, then datomic. |
So the idea of lookup refs is that they are resolved only when they appear at a statically known entity id positions. In this case you have no idea what Lookup refs are a bit of a hack added at later stages, that’s why they might behave inconsistently. They are resolved in some places Datomic authors thought about, but would simply not work in some others. |
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I see, that should make reasoning about them a bit simpler. Since you asked for my idea of correct behavior I would suggest to not throw either way. This would be a difference compared to datomic. Either way, I'd settle with a solution that behaves the same way when sending the lookup ref as an argument and when inlining it in the query. I'm open to creating a PR if you tell me your final decision and give some pointers what to poke at. |
There's a bunch of similarly looking issues from the past but none seem to be this particular case. A bad lookup ref inside the query throws an exception but the same query returns an empty set when the lookup ref is passed in as a parameter. Since the resulting query is identical I'd expect the semantics to be identical too.
% clj -Sdeps '{:deps {datascript {:mvn/version "0.18.8"}}}' Downloading: datascript/datascript/0.18.8/datascript-0.18.8.pom from https://repo.clojars.org/ Downloading: datascript/datascript/0.18.8/datascript-0.18.8.jar from https://repo.clojars.org/ Clojure 1.9.0 user=> (require '[datascript.core :as db]) nil user=> (def s {:id {:db/unique :db.unique/identity}}) #'user/s user=> (def c (db/create-conn s)) #'user/c user=> (db/transact c [{:id :a}]) #object[datascript.core$transact$reify__3612 0x7f2c995b {:status :ready, :val #datascript.db.TxReport{:db-before #datascript/DB {:schema {:id #:db{:unique :db.unique/identity}}, :datoms []}, :db-after #datascript/DB {:schema {:id #:db{:unique :db.unique/identity}}, :datoms [[1 :id :a 536870913]]}, :tx-data [#datascript/Datom [1 :id :a 536870913 true]], :tempids #:db{:current-tx 536870913}, :tx-meta nil}}] user=> (db/q '[:find ?id :where [[:id :b] :id ?id]] @c) ExceptionInfo Nothing found for entity id [:id :b] clojure.core/ex-info (core.clj:4739) user=> (db/q '[:find ?id :in $ ?idsel :where [?idsel :id ?id]] @c [:id :b]) #{}