Data Modeling

Use :db/ident for Enumerations

Many databases need to represent a set of enumerated values. In Datomic, it is idiomatic to represent enumerated values as entities with a :db/ident attribute.

For example, the transaction below defines an :artist/country attribute, and two initial enumerated values, :country/CA and :country/JP, ISO alpha-2 country codes for Canada and Japan.

[{:db/ident :artist/country
 :db/valueType :db.type/ref
 :db/cardinality :db.cardinality/one
 :db/doc "An artist's country of residence"}

{:db/ident :country/CA}

{:db/ident :country/JP}]

The use of :db/ident on the enumerated value entities makes it possible to refer to the entities using their identity keywords, :country/CA and :country/JP.

The transaction below creates an artist named Leonard Cohen with the country :country/CA.

[{:artist/name "Leonard Cohen"
 :artist/country :country/CA}]

The :artist/country attribute is of type :db.type/ref, so its value must be a reference to another entity. The :country/CA keyword can be used as a value because it is the identity of an entity, as specified by :db/ident in the previous transaction.

Attempting to use identity keywords that have not been transacted as referenced identities will result in an anomaly.

Next: Schema Limits