Datomic update examples against a social news database

Datomic News Updates

;; Datomic example code

;; demonstrates various update scenarios, using a news database
;; that contains stories, users, and upvotes

;; grab an in memory database
(use '[datomic.api :only (q db) :as d])
(def uri "datomic:mem://foo")
(d/create-database uri)
(def conn (d/connect uri))

;; schema for stories
(d/transact
 conn
 [{:db/id #db/id[:db.part/db]
   :db/ident :story/title
   :db/valueType :db.type/string
   :db/cardinality :db.cardinality/one
   :db/fulltext true
   :db/index true
   :db.install/_attribute :db.part/db}
  {:db/id #db/id[:db.part/db]
   :db/ident :story/url
   :db/valueType :db.type/string
   :db/cardinality :db.cardinality/one
   :db.install/_attribute :db.part/db}
  {:db/id #db/id[:db.part/db]
   :db/ident :story/slug
   :db/valueType :db.type/string
   :db/cardinality :db.cardinality/one
   :db.install/_attribute :db.part/db}])

;; schema for comments
(d/transact
 conn
 [{:db/id #db/id[:db.part/db]
   :db/ident :comments
   :db/valueType :db.type/ref
   :db/cardinality :db.cardinality/many
   :db/isComponent true
   :db.install/_attribute :db.part/db}
  {:db/id #db/id[:db.part/db]
   :db/ident :comment/body
   :db/valueType :db.type/string
   :db/cardinality :db.cardinality/one
   :db.install/_attribute :db.part/db}
  {:db/id #db/id[:db.part/db]
   :db/ident :comment/author
   :db/valueType :db.type/ref
   :db/cardinality :db.cardinality/one
   :db.install/_attribute :db.part/db}])

;; schema for users
(d/transact
 conn
 [{:db/id #db/id[:db.part/db]
   :db/ident :user/firstName
   :db/index true
   :db/valueType :db.type/string
   :db/cardinality :db.cardinality/one
   :db.install/_attribute :db.part/db}
  {:db/id #db/id[:db.part/db]
   :db/ident :user/lastName
   :db/index true
   :db/valueType :db.type/string
   :db/cardinality :db.cardinality/one
   :db.install/_attribute :db.part/db}
  {:db/id #db/id[:db.part/db]
   :db/ident :user/email
   :db/index true
   :db/unique :db.unique/identity
   :db/valueType :db.type/string
   :db/cardinality :db.cardinality/one
   :db.install/_attribute :db.part/db}
  {:db/id #db/id[:db.part/db]
   :db/ident :user/upVotes
   :db/valueType :db.type/ref
   :db/cardinality :db.cardinality/many
   :db.install/_attribute :db.part/db}])

;; adding new stories:
;;   create new ids with #db/id literals
(d/transact
 conn
 [{:db/id #db/id [:db.part/user]
   :story/title "Teach Yourself Programming in Ten Years"
   :story/url "http://norvig.com/21-days.html"}
  {:db/id #db/id [:db.part/user]
   :story/title "Clojure Rationale"
   :story/url "http://clojure.org/rationale"}
  {:db/id #db/id [:db.part/user]
   :story/title "Beating the Averages"
   :story/url "http://www.paulgraham.com/avg.html"}])

;; adding new user who upvotes existing stories:
;;   query to find existing story ids
;;   create new user id with an explicit #db/id literal
;;           e.g #db/id [:db.part/user -1]
;;   associate existing stories with the new user id

(def all-stories (q '[:find ?e :where [?e :story/url]] (db conn)))
(let [user-id #db/id [:db.part/user -1]]
  (d/transact
   conn
   (conj
    (map
     (fn [[story-id]] [:db/add user-id :user/upVotes story-id])
     all-stories)
    {:db/id user-id
     :user/email "[email protected]"
     :user/firstName "John"
     :user/lastName "Doe"})))

;; updating user's name (without touching upvotes or stories)
;; user/email is a :db.unique/identity attribute, so upsert is
;; implicit (you do not have to know the :db/id in advance)
(d/transact
 conn
 [{:user/email "[email protected]" ;; this finds the existing entity
   :db/id #db/id [:db.part/user]  ;; will be replaced by exiting id
   :user/firstName "Johnathan"}])

;; for variety, we will grab John's id and stop relying on upsert
(def john
  (->> (q '[:find ?e :where [?e :user/email "[email protected]"]]
          (db conn))
       ffirst))

;; grab the id of single upvote
(def johns-upvote-for-pg
  (->> (q '[:find ?story
            :in $ ?e
            :where [?e :user/upVotes ?story]
                   [?story :story/url "http://www.paulgraham.com/avg.html"]]
          (db conn)
          john)
       ffirst))

;; remove a single upvote (John doesn't like pg anymore? ...)
(d/transact
 conn
 [[:db/retract john :user/upVotes johns-upvote-for-pg]])

;; double check that john still has two upvotes
(-> (d/entity (db conn) john)
    (get :user/upVotes))

;; remove all of John's upvotes
;;    create a query that *makes transaction data*
;;    then submit the transaction at any later point
;;         possibly compsing it with other data
(def data-that-retracts-johns-upvotes
  (q '[:find ?op ?e ?a ?v
       :in $ ?op ?e ?a
       :where [?e ?a ?v]]
     (db conn)
     :db/retract
     john
     :user/upVotes))

(d/transact
 conn
 (seq data-that-retracts-johns-upvotes))

;; double check that john has no more upvotes
(-> (d/entity (db conn) john)
    (get :user/upVotes))

(def all-story-ids
  (->> (q '[:find ?e :where [?e :story/url]] (db conn))
       (map first)))

(defn choose-some
  "Pick zero or more items at random from a collection"
  [coll]
  (take (rand-int (count coll))
        (shuffle coll)))

;; sample data generator
(defn example-users
  "Make data for example users, possibly with upvotes"
  [prefix n]
  (mapcat
   (fn [n]
     (let [user-id (d/tempid :db.part/user)
           upvotes (map (fn [id] [:db/add user-id :user/upVotes id])
                        (choose-some all-story-ids))]
       (conj
        upvotes
        {:db/id user-id
         :user/email (str prefix "-" n "@example.com")})))
   (range n)))

;; test example-users
(pprint
 (example-users "user" 2))

;; make 10 new users
(d/transact conn (example-users "user" 10))

;; how many users should there be now? 
(count (q '[:find ?e :where [?e :user/email]] (db conn)))

;; how many users have upvoted something?
(count (q '[:find ?e
            :where [?e :user/email]
                   [?e :user/upVotes]]
          (db conn)))

;; find all users, and print their emails plus their upvotes
;;   (This shows why Datomic does not need left joins:
;;    query and entity navigation are separate.)
(let [dbval (db conn)]                                ;; shared basis
  (->> (q '[:find ?e :where [?e :user/email]]         ;; find all
          dbval)
       (map (fn [[id]] (d/entity dbval id)))          ;; entify
       (map (fn [ent]                                 ;; navigate
              {:email (:user/email ent)
               :upvoted (mapv :story/url (:user/upVotes ent))}))
       pprint))


;; schema for publication date
(d/transact
 conn
 [{:db/id #db/id[:db.part/db]
   :db/ident :publication/date
   :db/valueType :db.type/instant
   :db/cardinality :db.cardinality/one
   :db/index true
   :db.install/_attribute :db.part/db}])

;; find that pg story
(def beating-the-averages
  (->> (q '[:find ?e
            :where [?e :story/url "http://www.paulgraham.com/avg.html"]]
          (db conn))
       ffirst))


;; associate story with pub date
(import java.text.SimpleDateFormat)
(def april-04
  (-> (java.text.SimpleDateFormat. "yyyy-MM") (.parse "2001-04")))
(d/transact
 conn
 [[:db/add beating-the-averages :publication/date april-04]])

;; retrieve pub date (domain specific) and transaction time
;; (intrinsic to Datomic)
(let [[pub-time tx-time]
      (first (q '[:find ?pub-time ?tx-time
                  :in $ ?e
                  :where
                  [?e :story/url _ ?tx]
                  [?tx :db/txInstant ?tx-time]
                  [?e :publication/date ?pub-time]]
                (db conn)
                beating-the-averages))]
  (println "The essay _Beating the Averages was published " pub-time
           " but that fact was added to the database on " tx-time))

;; schema for information source 
(d/transact
 conn
 [{:db/id #db/id[:db.part/db]
   :db/ident :audit/app
   :db/valueType :db.type/keyword
   :db/cardinality :db.cardinality/one
   :db/index true
   :db.install/_attribute :db.part/db}])

;; use the db.part/tx partition to assert facts about the the
;; current transaction
(d/transact
 conn
 [{:db/id #db/id [:db.part/user]
   :story/title "Are We There Yet?"
   :story/url "http://www.infoq.com/presentations/Are-We-There-Yet-Rich-Hickey"}
  [:db/add (d/tempid :db.part/tx) :audit/app :admin/shell]])

;; find stories added by the admin/shell app
(q '[:find ?url
     :where [_ :story/url ?url ?tx]
            [?tx :audit/app :admin/shell]]
   (db conn))