Updating value with cardinality many - datomic

Question

I have a schema like this:
[{:db/id #db/id[:db.part/db]
:db/ident :person/name
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db/doc "A person's name"
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :person/roles
:db/valueType :db.type/keyword
:db/cardinality :db.cardinality/many
:db/doc "A person's role"
:db.install/_attribute :db.part/db}]
And a code like this:
;; insert new person
(def new-id (-> (d/transact conn [{:db/id (d/tempid :db.part/user)
:person/name "foo"
:person/roles #{:admin}}])
(:tempids)
(vals)
(first)))
(defn get-roles
[db eid]
(d/q '[:find ?roles .
:in $ ?eid
:where [?eid :user/roles ?roles]]))
(get-roles (d/db conn) new-id) ;; => [:admin]
;; update a person
(d/transact conn [{:db/id new-id
:person/roles #{:client}}])
(get-roles (d/db conn) new-id) ;; => [:admin :client]
It seems the default behaviour on it is, it will just assoc the new value.
How can I get this result, after doing the updating transaction:
(get-roles (d/db conn) new-id) ;; => [:client]

Answer 1

if what you want is to "reset" the list of roles to a new value (an 'absolute' operation 'in contrast to the 'relative' operations of just adding or removing roles), you'll have to use a transaction function to perform a diff and retract the values that need be.
Here's a basic generic implementation:
{:db/id (d/tempid :db.part/user),
:db/ident :my.fns/reset-to-many,
:db/fn
(d/function
{:lang :clojure,
:requires '[[datomic.api :as d]],
:params '[db e attr new-vals],
:code
'(let [ent (or (d/entity db e)
(throw (ex-info "Entity not found"
{:e e :t (d/basis-t db)})))
entid (:db/id ent)
old-vals (get ent attr)]
(into
[{:db/id (:db/id ent)
;; adding the new values
attr new-vals}]
;; retracting the old values
(comp
(remove (set new-vals))
(map (fn [v]
[:db/retract entid attr v])))
old-vals)
)})}
;; Usage
(d/transact conn [[:my.fns/reset-to-many new-id :person/roles #{:client}]])

Answer 2

Here is a solution once suggested from Robert Stuttaford
(defn many-delta-tx
"Produces the transaction necessary to have
`new` be the value for `entity-id` at `attr`
`new` is expected to be a set."
[db entity-id attr new]
(let [current (into #{} (map :v)
(d/datoms db :eavt
entity-id
(d/entid db attr)))]
(concat (for [id (clojure.set/difference new current)]
[:db/add entity-id attr id])
(for [id (clojure.set/difference current new)]
[:db/retract entity-id attr id]))))
For ease of testing, I would like to slightly modify part of the example of original question.
Change of the schema. I add db/unique
[{:db/id #db/id[:db.part/db]
:db/ident :person/name
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db/unique :db.unique/identity
:db/doc "A person's name"
:db.install/_attribute :db.part/db}
{:db/id #db/id[:db.part/db]
:db/ident :person/roles
:db/valueType :db.type/keyword
:db/cardinality :db.cardinality/many
:db/doc "A person's role"
:db.install/_attribute :db.part/db}]
get-roles
(defn get-roles
[db eid]
(d/q '[:find [?roles ...]
:in $ ?eid
:where [?eid :person/roles ?roles]] db eid))
My testing example
;; (many-delta-tx (d/db conn) [:person/name "foo"] :person/roles #{:sales :client})
;; => ([:db/add [:person/name "foo"] :person/roles :sales] [:db/retract [:person/name "foo"] :person/roles :admin]))