Are nested lookup refs in composite tuples supported in datomic? - clojure

Question

I'm using Datomic Ions to develop an application. In my schema I use composite tuples to guarantee uniqueness: shelves have books and the shelf+book combination must be unique. This is my schema:
{:db/ident :shelf/name
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db/unique :db.unique/value
:db/doc "A shelf is a grouping of books"}
;; Books
{:db/ident :book/shelf
:db/valueType :db.type/ref
:db/isComponent true
:db/cardinality :db.cardinality/one
:db/doc "Shelf this book belongs to"}
{:db/ident :book/id
:db/valueType :db.type/string
:db/cardinality :db.cardinality/one
:db/doc "The book identifier"}
;; Shelf + Book combination must be unique
{:db/ident :book/shelf+book
:db/valueType :db.type/tuple
:db/tupleAttrs [:book/shelf :book/id]
:db/cardinality :db.cardinality/one
:db/unique :db.unique/identity}
With the schema above I can do the following pull/queries:
(d/pull db '[*] [:shelf/name "my-shelf"])
Returns: {:db/id 74766790688854, :shelf/name "my-shelf"}
And:
(d/q '[:find ?b ?id
:in $ ?shelf+book
:where [?b :book/shelf+book ?shelf+book]
[?b :book/id ?id]]
db [74766790688854 "book-1"])
Returns: [[101155069755527 "book-1"]].
However I would like to use a lookup ref to resolve the shelf reference in a single query to avoid having to do a separate query to get the shelf reference, something like:
(d/q '[:find ?b ?id
:in $ ?shelf+book
:where
[?b :book/shelf+book ?shelf+book]
[?b :book/id ?id]]
db [[:shelf/name "my-shelf"] "book-1"])
But the above returns []. Is it possible to nest lookup refs like the above example?

Answer 1

I never tried this with composite tuples, but this is how it works in principle:
You can use the pull API inside normal queries like so:
(d/q '[:find ?id (pull ?b [:shelf/name])
:in $ ?shelf+book
:where
[?b :book/shelf+book ?shelf+book]
[?b :book/id ?id]]
db [[:shelf/name "my-shelf"] "book-1"])
I use this a lot for querying graphs inside datomic. Here's an example from my application:
(d/q '[:find
?id
(pull ?t [:db/ident])
(pull ?target-node [:my.graph.node/id
:my.graph.node/label])
:where
[?e :my.graph.edge/id ?id]
[?e :my.graph.edge/type ?t]
[?e :my.graph.edge/target ?target-node]]
db)
Where my.graph.edge/target is a :ref and my.graph.edge/type is the value of an enum.
An example output would be
[[#uuid"8e5fb3a4-1cac-40e2-ab8f-33352b6cabb3"
#:db{:ident :my.graph.edge.type/relationship}
#:my.graph.node{:label "Some node"}]
...]