Get all fields from a Datomic entity

Question

The 'Embedded' section of the Datomic Queries and Rules document says:

Query languages like SQL are oriented around a client-server model where, in a single conversaton, you are going to have to both:

• Answer your fundamental question, e.g. who bought socks this month.
• Recover any additional information required for reporting and processing, e.g. what are their names and email addresses.

The latter is not really a query, it is just a mechanical navigation to related information.

While I appreciate how the orthogonality of the two different mentioned aspects is honored, I think I'll often need to retrieve a whole entity, whichever its attributes are.

As far as I know, queries typically have this form:

(datomic.api/q '[:find ?name ?age ?email
                 :where
                 [?e :myapp/name ?name]
                 [?e :myapp/age ?age]
                 [?e :myapp/email ?email]]
               (db conn))

If I wanted to retrieve entities that have N attributes, I'd have them to list them all in each query, which seems tedious and error-prone to me.

How to tell Datomic to retrieve the entities with all the fields they were persisted with, without having to specify them explicitly?

Answer 1

Having the entity (id) from query like:

=> (def eid (d/q '[:find ?e :where [?e :myapp/name "Fred"]] (db conn)))

you can get the EntityMap:

=> (def ent (d/entity (db conn) (ffirst eid)))

so you can access fields/attributes without making additional query:

=> (seq ent)
;; ([:myapp/name "Fred"] [:myapp/age 16] [:myapp/email "[email protected]"])

however it may be easier to get the keys first:

=> (keys ent)
;; (:myapp/name :myapp/age :myapp/email)

You can even get reverse keys ("foreign" ref attributes that point to this entity) using the following trick:

=> (.touch ent)
=> (keys (.cache ent))

Answer 2

You can use pull to get all fields from an entity, or even just a selection. Using '[*] as the pattern for pull will retrieve all fields

See the pull documentation for more information.

To get all fields from an entity with id eid use:

(d/pull (db conn) '[*] eid)

Pull can also be used in queries:

(datomic.api/q '[:find (pull ?e [*])
                 :where
                 [?e :myapp/name]
               (db conn))