How to use / in a Datomic query?

Question

This query

[:find ?p ?af
 :where [?p :person/age ?pa]
        [?p :person/father ?f]
        [?f :person/age ?fa]
        [(/ ?pa ?fa) ?af]]

returns only rounded (to 0 decimal places) values for ?af, so I can't add sth. like

 [(< 0.25 ?af)]
 [(< ?af 0.5)]

to the :where clause.

I have tried casts to various types inside the query like

[(double ?pa) ?pa2]

but this only affects the result type and never removes the rounding.

If you know something and are experienced, I have quite a few more (especially type related) problems with Datomic. However I have not found a work-around for this division-query problem (except dividing after querying) and I really hope that it is not owed to the early development-stage of Datomic. I'd guess because Datomic does not support the Ratio type, they are rounding, but a few decimal places would be really nice.

I am using Datomic with Clojure.

EDIT I found a work-around for this, however I would still like to know why it is necessary.

Work-around:

• Use clojure.core//

  [:find ?p ?af
   :where [?p :person/age ?pa]
          [?p :person/father ?f]
          [?f :person/age ?fa]
          [(clojure.core// ?pa ?fa) ?af]]
  

But I certainly want to know which / is called (when invoked without a fully qualified namespace) and where I can find it's documentation so that I can learn about other possible problems with other operators.

EDIT2: Invoking an exception by designing a query to divide by zero, I found out that the static method clojure.lang.Numbers/quotient is invoked. How should I have known this? It seems like Datomic is not opensource, at least I could not find the source-code in the jar file. Using clojure.tools.trace on datomic.datalog, I found out that the invoke happens by resolving / from datomic.extensions. In datomic.extions, / is defined as clojure.core/quot, which invokes clojure.lang.Numbers/quotient.

All this is undocumented as far as I am concerned.

Answer 1

Indeed, it is an undocumented feature. Stuart Halloway has just confirmed this in the Datomic group:

Documentation for the operators available in query is missing, as you discovered. We are working on that, thanks for the report. The / operator currently performs integer division only, and we are looking into ways to make that richer without exposing consumers to a type (ratio) that most JVM languages do not have.

Regarding the correct workaround I will keep this answer updated.