28 September 2014
Datomic's new Pull API is a declarative way to make hierarchical selections of information about entities. You supply a pattern to specify which attributes of the entity (and nested entities) you want to pull, and db.pull returns a map for each entity.
;; Clojure API
(d/pull db '[*] led-zeppelin)
;; Java API
db.pull("[*]", ledZeppelin)
;; result
{:artist/sortName "Led Zeppelin",
:artist/name "Led Zeppelin",
:artist/type {:db/id 17592186045746},
:artist/country {:db/id 17592186045576},
:artist/gid #uuid "678d88b2-87b0-403b-b63d-5da7465aecc3",
:artist/endDay 25,
:artist/startYear 1968,
:artist/endMonth 9,
:artist/endYear 1980,
:db/id 17592186050305}
;; pattern
[:artist/name :artist/gid]
;; input
led-zeppelin
;; result
{:artist/gid #uuid "678d88b2-87b0-403b-b63d-5da7465aecc3",
:artist/name "Led Zeppelin"}
;; pattern
[:artist/_country]
;; input
greatBritain
;; result
{:artist/_country [{:db/id 17592186045751}
{:db/id 17592186045755}
...]}
;; pattern
[:release/media]
;; input
darkSideOfTheMoon
;; result
{:release/media
[{:db/id 17592186121277,
:medium/format {:db/id 17592186045741},
:medium/position 1,
:medium/trackCount 10,
:medium/tracks
[{:db/id 17592186121278,
:track/duration 68346,
:track/name "Speak to Me",
:track/position 1,
:track/artists [{:db/id 17592186046909}]}
{:db/id 17592186121279,
:track/duration 168720,
:track/name "Breathe",
:track/position 2,
:track/artists [{:db/id 17592186046909}]}
{:db/id 17592186121280,
:track/duration 230600,
:track/name "On the Run",
:track/position 3,
:track/artists [{:db/id 17592186046909}]}
...]}]}
;; pattern
[:track/name {:track/artists [:db/id :artist/name]}]
;; input
ghostRiders
;; result
{:track/artists [{:db/id 17592186048186, :artist/name "Bob Dylan"}
{:db/id 17592186049854, :artist/name "George Harrison"}],
:track/name "Ghost Riders in the Sky"}
;; pattern
[{:release/media
[{:medium/tracks
[:track/name {:track/artists [:artist/name]}]}]}]
;; input
concertForBanglaDesh
;; result
[{:medium/tracks
[{:track/artists
[{:artist/name "Ravi Shankar"} {:artist/name "George Harrison"}],
:track/name "George Harrison / Ravi Shankar Introduction"}
{:track/artists [{:artist/name "Ravi Shankar"}],
:track/name "Bangla Dhun"}]}
{:medium/tracks
[{:track/artists [{:artist/name "George Harrison"}],
:track/name "Wah-Wah"}
{:track/artists [{:artist/name "George Harrison"}],
:track/name "My Sweet Lord"}
{:track/artists [{:artist/name "George Harrison"}],
:track/name "Awaiting on You All"}
{:track/artists [{:artist/name "Billy Preston"}],
:track/name "That's the Way God Planned It"}]
...]}
