I want to use Datalevin as a database for my app. The README mentions :db/id as the identifier of entities, and I see these do get an autoincremented integer value on insertion.
However, there are also multiple mentions of :db/ident in the source code, for example for implicit constants.
What are the purposes of the two keywords and what are the differences?
:db/ident are called Entity
Identifiers
in Datomic.
They are used to allow for easier "pointing" to other, well known things; like an enum. So you can have the enum values as datums, but still be able to reference them via a keyword (no need to look them up every time before using them).
E.g.
(def conn (d/create-conn "./tst" {}))
(d/transact! conn
[{:db/id 1, :customer-type/name "Fictional", :db/ident :fictional}
{:db/id 2, :customer/name "ACME", :customer/type :fictional}])
(d/pull @conn
[:db/id :customer/name {:customer/type [:db/id :customer-type/name]}]
2)
; ⇒ {:db/id 2, :customer/name "ACME",
; :customer/type {:db/id :fictional,
; :customer-type/name "Fictional"}}