I recently encountered a slight difficulty with updating cardinality many attributes in Datomic, so I thought I would make a short walkthrough post about it.

The Problem

In Datomic each attribute has a “cardinality”, signifying how many values an attribute is allowed. Cardinality can be either “one” or “many”. Adding values for a cardinality many attribute is fairly straightforward, but updating them to a specific set is more difficult. Let’s take the example of a simple Todo list, where each todo has a title and multiple tags. Our schema looks like this:

(def schema
  [{:db/ident :todo/title
    :db/valueType :db.type/string
    :db/cardinality :db.cardinality/one}
   {:db/ident :todo/tags
    :db/valueType :db.type/string
    :db/cardinality :db.cardinality/many}])

Creating a new todo with a set of tags is fairly straightforward:

(d/transact
 conn
 [{:todo/title "Do the dishes"
   :todo/tags ["whenever"]}])

As is adding a tag to an existing todo:

(d/transact conn [:db/add todo-entity-id :todo/tags "boring"])

However, setting a todo’s tags to a specific set of values is more difficult. There’s no built-in way to do this in Datomic. Let’s say we want to set the tags for a todo to ["important" "today"], how would we do that?

The Solution

One solution is to:

1. Query the current values of the attribute
2. Diff them with the new values
3. Use that diff to create transactions to:
4. Add any new values
5. Retract values we no longer want
6. Apply the transactions using d/transact

Here’s the code to do this:

(defn update-attr-txs [db entity-id attr values]
  (let [;; Step 1
        current-vals (-> (d/q (vec
                                (concat
                                  '[:find [?a ...]]
                                   [:where ['?id attr '?a]]
                                  '[:in $ ?id]))
                              db
                              entity-id)
                         (set))
        ;; Step 2
        [added removed] (clojure.data/diff (set values) current-vals)]
    ;; Step 3
    (concat 
      ;; Step 3.1
      (->> added 
           (map #(-> [:db/add entity-id attr %])))
      ;; Step 3.2
      (->> removed
           (map #(-> [:db/retract entity-id attr %]))))))

 ;; Step 4
 (->> (update-attr-txs (d/db conn) todo-entity-id :todo/tags #{"important" "today"})
      (d/transact conn))

That should be all you need! I’ve created a gist containing the update-attr-txs function here . You can find some alternative solutions in this StackOverflow post , and also this one .

I hope this helps you out, please don’t hesitate to get in touch if you have any questions.