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Entity Specs

Similar to Datomic's entity specs Datahike supports assertion to ensure properties of transacted entities.

In short: you need to transact a spec to the database using :db/ident as the identifier, with at least either :db.entity/attrs as a list of attributes defined in the schema for ensuring required attributes, or :db.entity/preds with a list of fully namespaced symbols that refer to predicate function that you want to assert. The signature of the predicate function should be of [db eid] with the database record db where the transaction has happened and the entity eid to be asserted.


Imagine you have a database where an account has an email, a holder, and a balance. Each new account should have an email and a holder as a required attribute and the email should be checked for its standard. The next spec should check a new balance to not be less than 0.

Let's fire up a REPL:

(require '[datahike.api :as d])

;; setup database with schema and connection
(def schema [{:db/ident :account/email
              :db/valueType :db.type/string
              :db/unique :db.unique/identity
              :db/cardinality :db.cardinality/one}
             {:db/ident :account/holder
              :db/valueType :db.type/string
              :db/cardinality :db.cardinality/one}
             {:db/ident :account/balance
              :db/valueType :db.type/long
              :db/cardinality :db.cardinality/one}])

(def cfg {:store {:backend :mem
                  :id "accounts"}
          :name "accounts"
          :schema-flexibility :write
          :keep-history? true
          :initial-tx schema})

(d/delete-database cfg)

(d/create-database cfg)

(def conn (d/connect cfg))

;; define both predicates
(defn is-email? [db eid]
  (if-let [email (:account/email (d/entity db eid))]
    (-> (re-find #"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)" email) empty? not)

(defn positive-balance? [db eid]
  (if-let [balance (-> (d/entity db eid) :account/balance)]
    (< 0 balance)

;; add the person spec
(d/transact conn {:tx-data [{:db/ident :person/guard
                             :db.entity/attrs [:account/email :account/holder]
                             :db.entity/preds ['user/is-email?]}]})

(def valid-account {:account/email ""
                    :account/holder "Emma"})

;; add a valid person
(d/transact conn {:tx-data [(assoc valid-account :db/ensure :person/guard)]})

;; add with missing holder, observe exception
(d/transact conn {:tx-data [{:account/email ""
                             :db/ensure :person/guard}]})

;; add with invalid email, observe exception
(d/transact conn {:tx-data [{:account/email "thekla@datahike"
                             :account/holder "Thekla"
                             :db/ensure :person/guard}]})

;; add the balance spec
(d/transact conn {:tx-data [{:db/ident :balance/guard
                             :db.entity/attrs [:account/balance]
                             :db.entity/preds ['user/positive-balance?]}]})

;; add valid balance
(d/transact conn {:tx-data [{:db/id [:account/email (:account/email valid-account)]
                             :account/balance 1000
                             :db/ensure :balance/guard}]})

;; add invalid negative balance, observe exception
(d/transact conn {:tx-data [{:db/id [:account/email (:account/email valid-account)]
                             :account/balance -100
                             :db/ensure :balance/guard}]})

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