Datomic query for maximum of aggregated value

datalog, datomic

Solution

OK. Since I found an answer by myself - I am posting it here in case somebody will search for:

The solution is to build two datomic queries passing output of first one to second one.

(->>
  (d/q '[:find (count ?b) ?a :where [?a :author/books ?b]] db)
  (d/q '[:find (max ?count) ?a :in $ [?count ?a]] db))

This is as far as I got it the common way to work with less trivial queries in datomic - split it to several subqueries and chain together giving the DB do its job.

Problem

Assume I have entity `author` with many related `book` entities. What's the query to fetch author with biggest amount of books?

Original source