[Bug] `d/entity` throws Exception

[MrEbbinghaus]

From the docs of d/entity :

If entity does not exist, `nil` is returned:

          (entity db -1) ; => nil

But this is not the behavior. (At least with the file backend)

(d/entity @conn 1234567890) ; not in my db
=> #:db{:id 1234567890}
(d/entity @conn -1)
2021-07-05T14:50:30.244Z Jeb ERROR [datahike.db:1064] - Expected number or lookup ref for entity id, got  -1 {:error :entity-id/syntax, :entity-id -1}
Execution error (ExceptionInfo) at datahike.db/entid (db.cljc:1064).
Expected number or lookup ref for entity id, got -1

[kordano]

Thanks, we will adjust the documentation accordingly.

[TimoKramer]

Is this expected behaviour? I would expect to get a nil back? At least for a positive number I would expect that.

[jsmassa]

Yes, it sounds like the behaviour for positive numbers is definitely incorrect and for invalid input like negative numbers we either should adjust the documentation or the behaviour.

[MrEbbinghaus]

I just tried it with Datomic:

(d/entity db -1)
=> #:db{:id -1}
(def nonexistent-entity (d/entity db 2234234))
=> #:db{:id 2234234}
(:foo nonexistent-entity)
=> nil

It makes sense that you can create entities with nonexistent IDs, since attributes are looked up on demand. The Datomic docs even state:

Entities are not suitable for existence tests, which should typically be performed via lookup of a unique identity.

---

That said: datahike returns nil for nonexistent entities.

[TimoKramer]

Thanks @MrEbbinghaus for clarification. I was struck by something like this:

(d/entity db [:nonexistent "Foo"])

2023-06-29T06:32:13.243Z Zockolette ERROR [datahike.db.utils:110] - Lookup ref attribute should be marked as :db/unique:  [:nonexistent "Foo"] {:error :lookup-ref/unique, :entity-id [:nonexistent "Foo"}
Execution error (ExceptionInfo) at datahike.db.utils/entid (utils.cljc:110).
Lookup ref attribute should be marked as :db/unique: [:nonexistent "Foo"]

And I think this should not throw an Exception, like it should not throw with -1. So this is a bug as I see it. Right?

[TimoKramer]

To match Datomic behaviour it should return an entityID. But to fix this it should at least not throw.