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Datomic: containment relationships i.e. db/isComponent
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| (require '[datomic.api :as d]) | |
| (def uri "datomic:mem://test") | |
| (d/create-database uri) | |
| (def conn (d/connect uri)) | |
| (d/transact conn [;; Article | |
| {:db/id #db/id [:db.part/db] | |
| :db/ident :article/title | |
| :db/valueType :db.type/string | |
| :db/cardinality :db.cardinality/one | |
| :db/doc "This article's title" | |
| :db.install/_attribute :db.part/db} | |
| {:db/id #db/id [:db.part/db] | |
| :db/ident :article/comments | |
| :db/valueType :db.type/ref | |
| :db/cardinality :db.cardinality/many | |
| :db/isComponent true | |
| :db/doc "This article's comments" | |
| :db.install/_attribute :db.part/db} | |
| ;; Comment | |
| {:db/id #db/id [:db.part/db] | |
| :db/ident :comment/body | |
| :db/valueType :db.type/string | |
| :db/cardinality :db.cardinality/one | |
| :db/doc "This comment's body" | |
| :db.install/_attribute :db.part/db}]) | |
| (d/transact conn | |
| [;; A user writing an article | |
| {:db/id #db/id [:db.part/user -100] | |
| :article/title "Monads in Pictures" | |
| {:db/id #db/id [:db.part/user -200] | |
| :comment/body "Great article!" | |
| :article/_comments #db/id [:db.part/user -100]} | |
| {:db/id #db/id [:db.part/user -200] | |
| :comment/body "I like the pictures" | |
| :article/_comments #db/id [:db.part/user -100]}]) | |
| ;; Find article's comments | |
| (defn find-comments [db article-title] | |
| (d/q '[:find ?c | |
| :in $ ?a | |
| :where | |
| [?aid :article/title ?a] | |
| [?aid :article/comments ?cid] | |
| [?cid :comment/body ?c]] | |
| db article-title)) | |
| (defn get-eid [db attr-name attr-value] | |
| (ffirst (d/q '[:find ?e | |
| :in $ ?an ?av | |
| :where | |
| [?e ?an ?av]] | |
| db attr-name attr-value))) | |
| (def article-title "Monads in Pictures") | |
| (find-comments (d/db conn) article-title) | |
| ;; > #{["I like the pictures"] ["Great article!"]} | |
| (let [article-id (get-eid (d/db conn) :article/title article-title)] | |
| (d/transact conn [[:db.fn/retractEntity article-id]])) | |
| ;; NO comments are left after deleting the article because the | |
| ;; :article/comments attribute is defined as a containment | |
| ;; relationship i.e. db/isComponent true | |
| (find-comments (d/db conn) article-title) | |
| ;; > #{} |
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