Datomic Pull :as

05 November 2017

Datomic's Pull API provides a declarative way to make hierarchical and nested selections of information about entities.  The 0.9.5656 release enhances the Pull API with a new :as clause that provides control over the returned keys.

As an example, imagine that you want information about Led Zeppelin's tracks from the mbrainz dataset. The following pull pattern navigates to the artist's tracks, using limit to return a single track:

;; pull expression
'[[:track/_artists :limit 1]]

=> #:track{:_artists
 [#:db{:id 17592188757937}]}

;; pull pattern
'[{[:track/_artists :limit 1] [:track/name]}]

=> #:track{:_artists [#:track{:name "Black Dog"}]}

• you are targeting an environment that does not support symbolic names, so you need a string instead of a keyword key
• you do not want to expose the direction of navigation (e.g. the underscore in :track/_artists)
• your consumers are expecting a different name

;; pull expression
'[{[:track/_artists :limit 1 :as "Tracks"]
 [[:track/name :as "Name"]]}]

=> {"Tracks" [{"Name" "Black Dog"}]}