Working version of code from http://augustl.com/blog/2013/ordering_cardinality_many_in_datomic/

gistfile1.clj

{:db/id #db/id[:db.part/user]
 :db/ident :reset-position-in-scope
 :db/doc "Goes through existing positions and sequentializes them"
 :db/fn #db/fn {:lang "clojure"
                :params [db scope-id scope-attr retracted-eid sorted-attr]
                :code (let [children (scope-attr (datomic.api/entity db scope-id))
                           sorted-children-eids (map :db/id (sort-by sorted-attr children))
                           without-retracted-eids (filter (partial not= retracted-eid) sorted-children-eids)]
                       (map-indexed
                        (fn [idx entity-id]
                          [:db/add entity-id sorted-attr idx])
                        without-retracted-eids))}}