Unification with history db

[Marc Grue]

Hi,

I have an attribute :ns/str with an initial value "a" that is updated to
"b". So everything is fine when I use a history db to find the assertions
and
retractions over time:

[:find ?v ?op
:where [?e :ns/str ?v _ ?op]]
=>
["a" true]
["a" false]
["b" true]

I'm surprised though that ?v and "a" don't seem to unify in the following
query since I get back the exact same result:

[:find ?v ?op
:where [?e :ns/str ?v _ ?op]
[?e :ns/str "a"]]
=>
["a" true]
["a" false]
["b" true]

Why do I get back "b" also?

Thank you,
Marc

[Alan Thompson]

You are misunderstanding unification, which happens only on ?e here. Your
query reads "Find any entity ?e that has the attr/val pair of { :ns/str "a"
}. Then, for that ?e, find all values ?v and ops ?o using [?e :ns/str ?v _
?op].

However, if you did this query using the "current" db value after the
update from "a" -> "b" I would not expect it to select the entity in the
first place.

Alan

Post by Marc Grue
Hi,
I have an attribute :ns/str with an initial value "a" that is updated to
"b". So everything is fine when I use a history db to find the assertions
and
[:find ?v ?op
:where [?e :ns/str ?v _ ?op]]
=>
["a" true]
["a" false]
["b" true]
I'm surprised though that ?v and "a" don't seem to unify in the following
[:find ?v ?op
:where [?e :ns/str ?v _ ?op]
[?e :ns/str "a"]]
=>
["a" true]
["a" false]
["b" true]
Why do I get back "b" also?
Thank you,
Marc
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[Marc Grue]

Ah, of course. The same entity has attributes with any number of values
over time in a history db, not just a single ?v (or set).

This gives me what I want:

[:find ?v ?op
:where [?e :ns/str ?v _ ?op]
[(= ?v "a")]]
=>
["a" true]
["a" false]

Thanks,
Marc