如何构造一个完全匹配DataScript中refs向量的查询？

Question

设置考虑以下DataScript数据库的电影和演员,数据从learndatalogtoday.org窃取:以下代码可以在JVM/Clojure REPL或ClojureScript REPL中执行,只要project.clj包含[datascript "0.15.0"]作为依赖项.

(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)

描述简而言之,这个数据库中有两种实体 - 电影和演员(意图是不变的词) - 以及三种数据:

• 电影实体:( :film/name一个独特的字符串)
• 电影实体:( :film/cast多个参考)
• actor实体:( :actor/name唯一字符串)

问题我想构建一个问题:哪些电影有这些N演员,而这些N演员单独出现,作为唯一的星星,N> = 2？

例如,RoboCop主演南希艾伦,彼得韦勒,罗尼考克斯,但没有一部电影只出演前两部分,艾伦和韦勒.因此,我希望以下查询生成空集:

(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"]}

然而,查询是有缺陷的,因为我不知道如何表达任何比赛应该排除任何不是艾伦或韦勒的演员再次,我想找到只有艾伦和韦勒没有任何其他演员合作的电影,所以我想调整上面的查询来生成空集.如何调整此查询以强制执行此要求？

Answer 1

因为 DataScript 没有否定（截至 2016 年 5 月），所以我认为“纯”Datalog 中的一个静态查询不可能实现这一点。

我的方法是：

1. 以编程方式构建查询以添加 N 个子句，声明演员阵容必须包含 N 个演员
2. 添加一个谓词函数，在给定电影、数据库和演员 ID 集的情况下，该函数使用 EAVT 索引来查找每部电影是否有不在该集中的演员。

这是一个基本的实现

(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?)))