Tested on datascript 1.3.0
datoms:
[{:db/id -1 :name "Smith" :firstname "Oliver" :age 20}
{:db/id -2 :name "Jones" :firstname "Oliver" :age 20}
{:db/id -3 :name "Smith" :firstname "Amelia" :age 16}
{:db/id -4 :name "Jones" :firstname "Amelia" :age 16}]
tried to query with logical and predicates below who are named Smith and aged older than 18 years, why did it return the unfiltered whole set?
'[:find ?firstname ?name
:where
[?p :name ?name]
[?p :firstname ?firstname]
[?p :age ?age]
[(and (= ?name "Smith") (> ?age 18))]]
;;; wrong result: #{[Oliver Smith] [Oliver Jones] [Amelia Smith] [Amelia Jones]}
then changed to query with discrete predicates and got the satisfied result as expected.
'[:find ?firstname ?name
:where
[?p :name ?name]
[?p :firstname ?firstname]
[?p :age ?age]
[(= ?name "Smith")]
[(> ?age 18)]]
;;; correct result: #{[Oliver Smith]}
Do datomic and datascript or datalog in general only support data patterns scattered to discrete clauses? Are conventional logical operations and etc. incompatible here?
This is because AND is implicit. All clauses are implicitly joined by AND as in they all must be true at the same time for the query to match
According to the manual you cannot use an and-clause just like that. The only way you can use an and-clause is when it is inside an or-clause:
Inside the or clause, you may use an and clause to specify
conjunction. This clause is not available outside of an or clause,
since conjunction is the default in other clauses.
Related
Tested on datascript 1.3.0
datoms:
[{:db/id -1 :name "Oliver Smith" :hobbies ["reading" "sports" "music"]}]
tried to run the query below to find who like sports, but the empty set returned.
'[:find ?name
:where
[?p :name ?name]
[?p :hobbies ?hobbies]
[(some #{"sports"} ?hobbies)]]
How to formulate the query correctly to get the expected result below?
#{[Oliver Smith]}
We have to explicitly define the schema with cardinality/many against the attribute of multiple values to solve the problem since schemaless doesn't work here.
(require '[datascript.core :as d])
(def schema {:hobbies {:db/cardinality db.cardinality/many}})
(def conn (d/create-conn schema))
(def datoms [{:db/id -1 :name "Oliver Smith" :hobbies ["reading" "sports" "music"]}])
(d/transact! conn datoms)
(def query '[:find ?name :where [?p :name ?name] [?p :hobbies "sports"]])
(-> (d/q query #conn) println)
This question already has answers here:
Custom equality in Clojure distinct
(3 answers)
Closed 5 years ago.
For the sake of example, let's assume I have two sets:
(def set-a #{{:id 1 :name "ABC" :zip 78759} {:id 2 :name "DEF" :zip 78759}})
(def set-b #{{:id 1 :name "ABC" :zip 78753} {:id 3 :name "XYZ" :zip 78704}})
I would like to find an union between the sets, using only :id and :name fields. However, with out using a custom comparator I get four elements in the set, because :zip field is different.
(clojure.set/union set-a set-b)
#{{:id 3, :name "XYZ", :zip 78704} {:id 1, :name "ABC", :zip 78753}
{:id 1, :name "ABC", :zip 78759} {:id 2, :name "DEF", :zip 78759}}
What is the idomatic way of finding union between two sets using a custom comparator or compare?
You could use group-by to do this:
(map first (vals (group-by (juxt :id :name) (concat set-a set-b))))
Or threaded:
(->> (concat set-a set-b)
(group-by (juxt :id :name))
(vals)
(map first))
This is grouping your elements by a combination of their key/values i.e. (juxt :id :name). Then it grabs the values of the produced map, then maps first over that to get the first item in each grouping.
Or use some code specifically built for this like distinct-by.
Note these approaches apply to any collection, not just sets.
If you don't mind throwing :zip away entirely, consider using clojure.set/project.
(clojure.set/union
(clojure.set/project set-a [:id :name])
(clojure.set/project set-b [:id :name]))
#{{:id 3, :name "XYZ"} {:id 2, :name "DEF"} {:id 1, :name "ABC"}}
Is there a simple way in specter to collect all the structure satisfying a predicate ?
(./pull '[com.rpl/specter "1.0.0"])
(use 'com.rpl.specter)
(def data {:items [{:name "Washing machine"
:subparts [{:name "Ballast" :weight 1}
{:name "Hull" :weight 2}]}]})
(reduce + (select [(walker :weight) :weight] data))
;=> 3
(select [(walker :name) :name] data)
;=> ["Washing machine"]
How can we get all the value for :name, including ["Ballast" "Hull"] ?
Here's one way, using recursive-path and stay-then-continue to do the real work. (If you omit the final :name from the path argument to select, you'll get the full “item / part maps” rather than just the :name strings.)
(def data
{:items [{:name "Washing machine"
:subparts [{:name "Ballast" :weight 1}
{:name "Hull" :weight 2}]}]})
(specter/select
[(specter/recursive-path [] p
[(specter/walker :name) (specter/stay-then-continue [:subparts p])])
:name]
data)
;= ["Washing machine" "Ballast" "Hull"]
Update: In answer to the comment below, here's a version of the above the descends into arbitrary branches of the tree, as opposed to only descending into the :subparts branch of any given node, excluding :name (which is the key whose values in the tree we want to extract and should not itself be viewed as a branching off point):
(specter/select
[(specter/recursive-path [] p
[(specter/walker :name)
(specter/stay-then-continue
[(specter/filterer #(not= :name (key %)))
(specter/walker :name)
p])])
:name]
;; adding the key `:subparts` with the value [{:name "Foo"}]
;; to the "Washing machine" map to exercise the new descent strategy
(assoc-in data [:items 0 :subparts2] [{:name "Foo"}]))
;= ["Washing machine" "Ballast" "Hull" "Foo"]
The selected? selector can be used to collect structures for which another selector matches something within the structure
From the examples at https://github.com/nathanmarz/specter/wiki/List-of-Navigators#selected
=> (select [ALL (selected? [(must :a) even?])] [{:a 0} {:a 1} {:a 2} {:a 3}])
[{:a 0} {:a 2}]
I think you could iterate on map recursively using clojure.walk package. On each step, you may check the current value for a predicate and push it into an atom to collect the result.
Setup Consider the following DataScript database of films and cast, with data stolen from learndatalogtoday.org: the following code can be executed in a JVM/Clojure REPL or a ClojureScript REPL, as long as project.clj contains [datascript "0.15.0"] as a dependency.
(ns user
(:require [datascript.core :as d]))
(def data
[["First Blood" ["Sylvester Stallone" "Brian Dennehy" "Richard Crenna"]]
["Terminator 2: Judgment Day" ["Linda Hamilton" "Arnold Schwarzenegger" "Edward Furlong" "Robert Patrick"]]
["The Terminator" ["Arnold Schwarzenegger" "Linda Hamilton" "Michael Biehn"]]
["Rambo III" ["Richard Crenna" "Sylvester Stallone" "Marc de Jonge"]]
["Predator 2" ["Gary Busey" "Danny Glover" "Ruben Blades"]]
["Lethal Weapon" ["Gary Busey" "Mel Gibson" "Danny Glover"]]
["Lethal Weapon 2" ["Mel Gibson" "Joe Pesci" "Danny Glover"]]
["Lethal Weapon 3" ["Joe Pesci" "Danny Glover" "Mel Gibson"]]
["Alien" ["Tom Skerritt" "Veronica Cartwright" "Sigourney Weaver"]]
["Aliens" ["Carrie Henn" "Sigourney Weaver" "Michael Biehn"]]
["Die Hard" ["Alan Rickman" "Bruce Willis" "Alexander Godunov"]]
["Rambo: First Blood Part II" ["Richard Crenna" "Sylvester Stallone" "Charles Napier"]]
["Commando" ["Arnold Schwarzenegger" "Alyssa Milano" "Rae Dawn Chong"]]
["Mad Max 2" ["Bruce Spence" "Mel Gibson" "Michael Preston"]]
["Mad Max" ["Joanne Samuel" "Steve Bisley" "Mel Gibson"]]
["RoboCop" ["Nancy Allen" "Peter Weller" "Ronny Cox"]]
["Braveheart" ["Sophie Marceau" "Mel Gibson"]]
["Mad Max Beyond Thunderdome" ["Mel Gibson" "Tina Turner"]]
["Predator" ["Carl Weathers" "Elpidia Carrillo" "Arnold Schwarzenegger"]]
["Terminator 3: Rise of the Machines" ["Nick Stahl" "Arnold Schwarzenegger" "Claire Danes"]]])
(def conn (d/create-conn {:film/cast {:db/valueType :db.type/ref
:db/cardinality :db.cardinality/many}
:film/name {:db/unique :db.unique/identity
:db/cardinality :db.cardinality/one}
:actor/name {:db/unique :db.unique/identity
:db/cardinality :db.cardinality/one}}))
(def all-datoms (mapcat (fn [[film actors]]
(into [{:film/name film}]
(map #(hash-map :actor/name %) actors)))
data))
(def all-relations (mapv (fn [[film actors]]
{:db/id [:film/name film]
:film/cast (mapv #(vector :actor/name %) actors)}) data))
(d/transact! conn all-datoms)
(d/transact! conn all-relations)
Description In a nutshell, there are two kinds of entities in this database—films and actors (word intended to be ungendered)—and three kinds of datoms:
film entity: :film/name (a unique string)
film entity: :film/cast (multiple refs)
actor entity: :actor/name (unique string)
Question I would like to construct a query which asks: which films have these N actors, and these N actors alone, appeared as the sole stars, for N>=2?
E.g., RoboCop starred Nancy Allen, Peter Weller, Ronny Cox, but no film starred solely the first two of these, Allen and Weller. Therefore, I would expect the following query to produce the empty set:
(d/q '[:find ?film-name
:where
[?film :film/name ?film-name]
[?film :film/cast ?actor-1]
[?film :film/cast ?actor-2]
[?actor-1 :actor/name "Nancy Allen"]
[?actor-2 :actor/name "Peter Weller"]]
#conn)
; => #{["RoboCop"]}
However, the query is flawed because I don't know how to express that any matches should exclude any actors who are not Allen or Weller—again, I want to find the movies where only Allen and Weller have collaborated without any other actors, so I want to adapt the above query to produce the empty set. How can I adjust this query to enforce this requirement?
Because DataScript doesn't have negation (as of May 2016), I don't believe that's possible with one static query in 'pure' Datalog.
My way to go would be:
build the query programmatically to add the N clauses that state that the cast must contain the N actors
Add a predicate function which, given a movie, the database, and the set of actors ids, uses the EAVT index to find if each movie has an actor that is not in the set.
Here's a basic implementation
(defn only-those-actors? [db movie actors]
(->> (datoms db :eavt movie :film/cast) seq
(every? (fn [[_ _ actor]]
(contains? actors actor)))
))
(defn find-movies-with-exact-cast [db actors-names]
(let [actors (set (d/q '[:find [?actor ...] :in $ [?name ...] ?only-those-actors :where
[?actor :actor/name ?name]]
db actors-names))
query {:find '[[?movie ...]]
:in '[$ ?actors ?db]
:where
(concat
(for [actor actors]
['?movie :film/cast actor])
[['(only-those-actors? ?db ?movie ?actors)]])}]
(d/q query db actors db only-those-actors?)))
You can use predicate fun and d/entity together for filtering datoms by :film/cast field of an entity. This approach looks much more straightforward until Datascript doesn't support negation (not operator and so on).
Look at the row (= a (:age (d/entity db e)) in the test case of the Datascript here
[{:db/id 1 :name "Ivan" :age 10}
{:db/id 2 :name "Ivan" :age 20}
{:db/id 3 :name "Oleg" :age 10}
{:db/id 4 :name "Oleg" :age 20}]
...
(let [pred (fn [db e a]
(= a (:age (d/entity db e))))]
(is (= (q/q '[:find ?e
:in $ ?pred
:where [?e :age ?a]
[(?pred $ ?e 10)]]
db pred)
#{[1] [3]})))))
In your case, the predicate body could look something like this
(clojure.set/subset? actors (:film/cast (d/entity db e))
In regards to performance, the d/entity call is fast because it is a lookup by index.
I have 2 data structures like the ones below
(ns test)
(def l
[{:name "Sean" :age 27}
{:name "Ross" :age 27}
{:name "Brian" :age 22}])
(def r
[{:owner "Sean" :item "Beer" }
{:owner "Sean" :item "Pizza"}
{:owner "Ross" :item "Computer"}
{:owner "Matt" :item "Bike"}])
I want to have get persons who dont own any item . (Brian in this case so [ {:name "Brian" :age 22}]
If this was SQL I would do left outer join or not exists but I not sure how to do this in clojure in more performant way.
While Chuck's solution is certainly the most sensible one, I find it interesting that it is possible to write a solution in terms of relational algebraic operators using clojure.set:
(require '[clojure.set :as set])
(set/difference (set l)
(set/project (set/join r l {:owner :name})
#{:name :age}))
; => #{{:name "Brian", :age 22}}
You basically want to do a filter on l, but negative. We could just not the condition, but the remove function already does this for us. So something like:
(let [owner-names (set (map :owner r))]
(remove #(owner-names (% :name)) l))
(I think it reads more nicely with the set, but if you want to avoid allocating the set, you can just do (remove (fn [person] (some #(= (% :owner) (person :name)) r)) l).)