frinj unit conversion running inside a Datomic datalog query

gistfile1.clj

;; lein settings
(defproject foo "1.0.0-SNAPSHOT"
  :description "Test App"
  :dependencies [[com.datomic/datomic "0.1.2678"]
                 [frinj "0.1.2" :exclusions [org.clojure/clojure]]])

;; load libs
(use 'frinj.core 'frinj.calc)
(frinj-init!)
(use '[datomic.api :only (q db) :as d])

;; get an in-memory db
(def uri "datomic:mem://roster")
(d/create-database uri)
(def conn (d/connect uri))

;; teach Datomic that attributes can have units
(d/transact conn [{:db/id (d/tempid :db.part/db)
                   :db/ident :units
                   :db/valueType :db.type/keyword
                   :db/cardinality :db.cardinality/one
                   :db.install/_attribute :db.part/db}])

;; add schema types, some bearing units
(d/transact conn [{:db/id (d/tempid :db.part/db)
                   :db/ident :name
                   :db/valueType :db.type/string
                   :db/cardinality :db.cardinality/one
                   :db.install/_attribute :db.part/db}
                  {:db/id (d/tempid :db.part/db)
                   :db/ident :weight
                   :db/valueType :db.type/long
                   :units :pounds
                   :db/cardinality :db.cardinality/one
                   :db.install/_attribute :db.part/db}
                  {:db/id (d/tempid :db.part/db)
                   :db/ident :height
                   :units :inches
                   :db/valueType :db.type/long
                   :db/cardinality :db.cardinality/one
                   :db.install/_attribute :db.part/db}])

;; add a few values
(d/transact conn [{:db/id (d/tempid :db.part/user)
                   :name "Cole Aldrich"
                   :height 83
                   :weight 245}
                  {:db/id (d/tempid :db.part/user)
                   :name "Kevin Durant"
                   :height 81
                   :weight 215}])

;; adapter between frinj and query
(defn convert
  [& items]
  (-> (apply fj items) :v double))

;; Find players, converting height/weight from database rep 
;; to units requested. 
(defn roster
  [height-units weight-units]
  (q '[:find ?name ?m ?kg
       :in $ ?to-h ?to-w
       :where
       [?e :name ?name]
       [?e :height ?height]
       [:height :units ?from-h]
       [?e :weight ?weight]
       [:weight :units ?from-w]
       [(user/convert ?height ?from-h :to ?to-h) ?m]
       [(user/convert ?weight ?from-w :to ?to-w) ?kg]]
     (db conn)
     height-units
     weight-units))

;; get roster with some different units
(roster :meters :kilograms)
(roster :yards :stone)