Datomic Cheat Sheet Series - Queries (Part 4) - Pull

Software Engineering Team Lead and Director of Cloudsure

Thursday, 20 September 2018 · Estimated 3 minute read

In this post I work through pull expressions using the Datomic MusicBrainz sample database. The example queries are grabbed from the Datomic Docs. If you want to get started with Datomic, check out the first post in the series. If you want to see other queries, then check out part 1 and part 2. If you want to set up query rules then check out part 3.

Relationship diagram

Create a database connection

Create an inline database connection

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(require '[datomic.api :as d])
(def db-uri "datomic:dev://localhost:4334/mbrainz-1968-1973")
(def conn (d/connect db-uri))
(def db (d/db conn))

Create a database connection as a function

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(ns beryllium.core
  (:require [datomic.api :as d]))

(defn new-db []
  (let [db-uri "datomic:dev://localhost:4334/mbrainz-1968-1973"
        conn (d/connect db-uri)
        db (d/db conn)]
    db))

Example pull expressions

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(def artist-id (:db/id (first (d/q '[:find ([pull ?e [:db/id] ...])
                                     :in $ ?name
                                     :where [?e :artist/name ?name]] db "The Jimi Hendrix Experience"))))
; => #'user/artist-id

artist-id
; => 17592186048480

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(d/q '[:find ([pull ?e [*] ...])
       :in $ ?artist
       :where [?e :release/artists ?artist]] db artist-id)

; =>
; [{:release/name "1968-10-12: Winterland, San Francisco, CA, USA (Early Show)",
;   :release/artists [#:db{:id 17592186048480}],
;   :release/country #:db{:id 17592186045504},
;   ...}]

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(d/q '[:find ([pull ?e [:release/name] ...])
       :in $ ?artist
       :where [?e :release/artists ?artist]] db artist-id)

; => [#:release{:name "1968-10-12: Winterland, San Francisco, CA, USA (Early Show)"}]

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(d/q '[:find ([pull ?e [{:release/country [*]}] ...])
       :in $ ?artist
       :where [?e :release/artists ?artist]] db artist-id)

;=> [#:release{:country {:db/id 17592186045504, :db/ident :country/US, :country/name "United States"}}]

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(d/q '[:find ([pull ?e [{:release/artists [{:artist/country [*]}]}] ...])
       :in $ ?artist
       :where [?e :release/artists ?artist]] db artist-id)

; =>
; [#:release{:artists [#:artist{:country {:db/id 17592186045576, :db/ident :country/GB, :country/name "United Kingdom"}}]}]

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(d/q '[:find (pull ?e pattern)
       :in $ ?artist pattern
       :where [?e :release/artists ?artist]] db artist-id [:release/name :release/status])

; =>
; [[#:release{:name "1968-10-12: Winterland, San Francisco, CA, USA (Early Show)", :status "Bootleg"}]
;  [#:release{:name "Electric Ladyland", :status "Official"}]
;  [#:release{:name "Axis: Bold as Love", :status "Official"}]
;  ...]

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(d/q '[:find (pull ?e [:release/name])
       :in $ ?artist-name
       :where
       [?e :release/artists ?a]
       [?a :artist/name ?artist-name]] db "Led Zeppelin")

; =>
; [[#:release{:name "Immigrant Song / Hey Hey What Can I Do"}]
;  [#:release{:name "Heartbreaker / Bring It On Home"}]
;  [#:release{:name "Led Zeppelin III"}]
;  ...]

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(d/q '[:find (pull ?e [:release/name]) (pull ?a [*])
       :in $ ?artist-name
       :where
       [?e :release/artists ?a]
       [?a :artist/name ?artist-name]] db "Led Zeppelin")

; =>
; [[#:release{:name "Led Zeppelin II"}
;   {: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}]
;    ...]

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(d/q '[:find (pull ?e [:release/name :release/artists])
       :in $ ?artist-name
       :where [?e :release/artists ?a]
       [?a :artist/name ?artist-name]] db "Led Zeppelin")

;=>
; [[#:release{:name "Immigrant Song / Hey Hey What Can I Do", :artists [#:db{:id 17592186050305}]}]
;  [#:release{:name "Heartbreaker / Bring It On Home", :artists [#:db{:id 17592186050305}]}]
;  [#:release{:name "Led Zeppelin III", :artists [#:db{:id 17592186050305}]}]
;  ...]

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(d/pull db '[*] artist-id)

; =>
; {:artist/sortName "Hendrix, Jimi, The, Experience",
;  :artist/name "The Jimi Hendrix Experience",
;  :artist/type #:db{:id 17592186045746},
;  :artist/country #:db{:id 17592186045576},
;  :artist/gid #uuid"33b3c323-77c2-417c-a5b4-af7e6a111cc9",
;  :artist/startYear 1966,
;  :artist/endMonth 6,
;  :artist/endYear 1969,
;  :db/id 17592186048480}

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(d/pull db '[:artist/name :artist/gid] artist-id)

; => #:artist{:name "The Jimi Hendrix Experience", :gid #uuid"33b3c323-77c2-417c-a5b4-af7e6a111cc9"}