I'm trying to do an 'outer join' in Datomic via the REST API. From https://github.com/Datomic/day-of-datomic/blob/master/tutorial/social_news.clj I have taken the final example:
(defn maybe
"Returns the set of attr for e, or nil if e does not possess
any values for attr."
[db e attr]
(seq (map :a (d/datoms db :eavt e attr))))
;; find all users
(q '[:find ?e ?upvote
:where
[?e :user/email]
[(user/maybe $ ?e :user/upVotes) ?upvote]]
(db conn))
I inserted the maybe function into my database, and it can be queried thus:
[:find ?n ?v :in $ :where [?e ?a ?v] [?a :db/ident :db/fn] [?e :db/ident ?n]]
returns
:maybe #db/fn{:code "(seq (map :a (d/datoms db :eavt e attr)))", :params [db e attr], :requires [], :imports [], :lang :clojure}
However, I am unable to work out how to call the function in a query. I have an :data/user attribute on some transactions, which I want to get the value for where it exists. Here's the query I'm trying to run; I would like :maybe to be the database function defined above.
[:find ?attr ?v ?when ?who :where
[17592186045423 ?a ?v ?tx true]
[?a :db/ident ?attr]
[(:maybe $ ?tx :data/user) ?who]
[?tx :db/txInstant ?when]]
I'm pretty sure I'm missing something pretty obvious, but I've been stuck on this for a day now. Thanks for any help!
You need to use d/invoke. So your example would look like this:
[:find ?attr ?v ?when ?who :where
[17592186045423 ?a ?v ?tx true]
[?a :db/ident ?attr]
[(d/invoke $ :maybe ?tx :data/user) ?who]
[?tx :db/txInstant ?when]]