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Datomic queries against Clojure collections
;; Datomic example code
(use '[datomic.api :only (db q) :as d])
;; ?answer binds a scalar
(q '[:find ?answer :in ?answer]
;; of course you can bind more than one of anything
(q '[:find ?last ?first :in ?last ?first]
"Doe" "John")
;; [?last ?first] binds a tuple
(q '[:find ?last ?first :in [?last ?first]]
["Doe" "John"])
;; [?first ...] binds a collection
(q '[:find ?first
:in [?first ...]]
["John" "Jane" "Phineas"])
;; [[?first ?last]] binds a relation
(q '[:find ?first
:in [[?first ?last]]]
[["John" "Doe"]
["Jane" "Doe"]])
;; a database binding name starts with $ instead of ?
;; any relation with 4-tuples E/A/V/T can act as a database
;; so in Datomic, you can mock a database with a list of lists
(q '[:find ?first
:in $db
:where [$db _ :firstName ?first]]
[[1 :firstName "John"]])
;; same as previous, but omit $db for single-database query
;; any relation with 4-tuples eavt can act as a database
(q '[:find ?first
:where [_ :firstName ?first]]
[[1 :firstName "John" 42]
[1 :lastName "Doe" 42]])
;; simple in-memory join, two tuple bindings
(q '[:find ?first ?height
:in [?last ?first ?email] [?email ?height]]
["Doe" "John" ""]
["" 71])
;; simple in-memory join, two relation bindings
;; see next example for a faster approach
(q '[:find ?first ?height
:in [[?last ?first ?email]] [[?email ?height]]]
[["Doe" "John" ""]
["Doe" "Jane" ""]]
[["" 73]
["" 71]])
;; same as previous example, but with database expressions
;; runs faster than relation bindings (as of July 2012)
(q '[:find ?first ?height
:in $a $b
:where [$a ?last ?first ?email]
[$b ?email ?height]]
[["Doe" "John" ""]
["Doe" "Jane" ""]]
[["" 73]
["" 71]])
;; simple in-memory join, two database bindings
(q '[:find ?first ?height
:in $db1 $db2
:where [$db1 ?e1 :firstName ?first]
[$db1 ?e1 :email ?email]
[$db2 ?e2 :email ?email]
[$db2 ?e2 :height ?height]]
[[1 :firstName "John"]
[1 :email ""]
[2 :firstName "Jane"]
[2 :email ""]]
[[100 :email ""]
[100 :height 73]
[101 :email ""]
[101 :height 71]])
;; compare to
(q '[:find ?car ?speed
:in [[?car ?speed]]
:where [(> ?speed 100)]]
[["Stock" 225]
["Spud" 80]
["Rocket" 400]
["Stock" 225]
["Clunker" 40]])
;; compare to
(->> (q '[:find ?k ?v
:in [[?k ?v] ...]]
{:D 67.3 :A 99.5 :B 67.4 :C 67.5})
(sort-by second))

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@fogus fogus commented May 9, 2012

The age of the question-mark prefixed symbol in Clojure is nigh!


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@dhruv-dave dhruv-dave commented Oct 27, 2014

I like the idea of using datomic queries over clojure collections.
I am not sure how to use it to solve my problem.
I am not sure if I am approaching this problem correctly/appropriately.

(def collection [{:key-1 :val-a1 :key-2 :val-a2 ... :key-100 :val-a100}
{:key-1 :val-b1 :key-2 :val-b2 ... :key-100 :val-b100}
{:key-1 :val-x1 :key-2 :val-x2 ... :key-100 :val-x100}
{:key-1 :val-z1 :key-2 :val-z2 ... :key-100 :val-z100}])

(defn find-all
"Return the hash where the value for :key-99 is equal to :val-x100."
(d/q '[:find ?c
:in [?c ...]
[(-> ?c :key-100 (= :val-x100)) ?f]
[(= ?f true)]]

(find-all collection)

{[{:key-100 :val-x100, :key-1 :val-x1, :key-2 :val-x2}]}

;; I need to parameterize the value and pass it in as an argument.
;; Currently this does not work.

(defn find-all2
"Return the hash where the value for :key-99 is equal to 'value'."
[value collection]
(d/q '[:find ?c
:in ?v [?c ...]
[(-> ?c :key-100 (= ?v)) ?f]
[(= ?f true)]]
value collection))

(find-all2 :val-x100 collection)
CompilerException java.lang.RuntimeException: Unable to resolve symbol: ?v in this context, compiling:(/private/var/folders/0f/4w3qyc8s5vs9bnv6lfh5pg_80000gn/T/form-init8853993715126212403.clj:7:64)


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@timcreasy timcreasy commented Jan 14, 2020


Has there been any discussion about extending datomic.client.api/q to allow this sort of query behavior in Datomic Cloud?

(d/q '[:find ?first ?height
       :in $a $b
       :where [$a ?last ?first ?email]
       [$b ?email ?height]]
     [["Doe" "John" ""]
      ["Doe" "Jane" ""]]
     [["" 73]
      ["" 71]])
Execution error (ExceptionInfo) at datomic.client.api.impl/incorrect (impl.clj:42).
Query args must include a database

Looks like collections don't satisfy the Queryable protocol and thus fail.

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