# Conj in a rule fails on the transactor **URL:** \ **Category:** Datomic Applications\ **Created:** [April 24, 2018, 1:24am UTC](https://forum.datomic.com/t/conj-in-a-rule-fails-on-the-transactor/413 "2018-04-24T01:24:12Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![benfle](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.datomic.com/benfle/32/164_2.png) [@benfle](https://forum.datomic.com/u/benfle)\ **Post date:** [April 24, 2018, 1:24am UTC](https://forum.datomic.com/t/conj-in-a-rule-fails-on-the-transactor/413/1 "2018-04-24T01:24:12Z")
Hi everyone, I have a hard time understanding why the following rule sometimes fail with a ClassCastException on the `conj`. Apparently `?parent-path` is a Java array and not a IPersistentCollection. This only happens in the transactor and not with `d/with`. We would like to refer to nodes in a tree using their path from the root. I was hoping that a recursive rule like this would work: ``` (def node-path '[;; node without a parent [(node-path ?n ?p) [?n :node/name ?name] [(missing? $ ?n :node/parent)] [(vector ?name) ?p]] ;; node with a parent [(node-path ?n ?p) [?n :node/name ?name] [?n :node/parent ?parent] (node-path ?parent ?parent-path) [(conj ?parent-path ?name) ?p]]]) ``` I can send a minimal schema and query to reproduce my use-case if needed. My expectation was that a vector would always be built at the bottom of the recursion. `?p` would be bound to the result of `(vector ?name)`. And then we `conj` this vector until we reach the leaf. Was my expectation correct? If not, where is it wrong? Thanks a lot for your help. ---
**Author:** ![benfle](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.datomic.com/benfle/32/164_2.png) [@benfle](https://forum.datomic.com/u/benfle)\ **Post date:** [April 25, 2018, 12:58pm UTC](https://forum.datomic.com/t/conj-in-a-rule-fails-on-the-transactor/413/2 "2018-04-25T12:58:36Z")
To repro on the lastest Datomic free transactor. ``` (ns sandbox (:require [datomic.api :as d])) (def db-uri "datomic:free://localhost:4334/sandbox") (def schema [#:db{:ident :node/name :valueType :db.type/string :cardinality :db.cardinality/one} #:db{:ident :node/parent :valueType :db.type/ref :cardinality :db.cardinality/one} #:db{:ident :node/path->eid :fn #db/fn {:lang "clojure" :params [db path] :requires [[datomic.api :as d]] :code (ffirst (d/q '[:find ?e :in $ % ?path :where (node-path ?e ?path)] db '[[(node-path ?n ?p) [?n :node/name ?name] [(missing? $ ?n :node/parent)] [(vector ?name) ?p]] [(node-path ?n ?p) [?n :node/name ?name] [?n :node/parent ?parent] (node-path ?parent ?parent-path) [(conj ?parent-path ?name) ?p]]] path))}} #:db{:ident :node/ensure-path :fn #db/fn {:lang "clojure" :params [db path parent?] :requires [[datomic.api :as d]] :code (if-let [eid (d/invoke db :node/path->eid db path)] (when parent? eid) (let [name (last path) parent-path (butlast path)] [(cond-> {:db/id (d/tempid :db.part/user) :node/name name} (seq parent-path) (assoc :node/parent (d/invoke db :node/ensure-path db parent-path true)))]))}}]) (defn connect [] (d/create-database db-uri) (let [conn (d/connect db-uri)] @(d/transact conn schema) conn)) (def test-data [["1" ["1A"] ["1B" ["1B1"] ["1B2"]]] ["2"]]) (defn plex-tx ([data] (plex-tx data nil)) ([data parent] (mapcat (fn [[name & children]] (let [tempid (d/tempid :db.part/user) node (cond-> {:db/id tempid :node/name name} parent (assoc :node/parent parent))] (into [node] (plex-tx children tempid)))) data))) (comment (def conn (connect)) @(d/transact conn (plex-tx test-data)) (def db (d/db conn)) (d/invoke db :node/path->eid db ["1" "1B" "1B1"]) @(d/transact conn [[:node/ensure-path ["1" "1B" "1B3"] false]]) @(d/transact conn [[:node/ensure-path ["1" "1B"] false]]) ;; This tx throws @(d/transact conn [[:node/ensure-path ["1"] false]]) ) ``` ---
**Author:** ![dustingetz](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.datomic.com/dustingetz/32/56_2.png) [@dustingetz](https://forum.datomic.com/u/dustingetz)\ **Post date:** [December 21, 2018, 3:59pm UTC](https://forum.datomic.com/t/conj-in-a-rule-fails-on-the-transactor/413/3 "2018-12-21T15:59:41Z")
Bump! ---
**Author:** ![marshall](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.datomic.com/marshall/32/48_2.png) [@marshall](https://forum.datomic.com/u/marshall)\ **Post date:** [January 14, 2019, 4:53pm UTC](https://forum.datomic.com/t/conj-in-a-rule-fails-on-the-transactor/413/4 "2019-01-14T16:53:39Z")
This is a consequence of a couple factors: - array lists and vectors compare equal - you can’t force something to be a vector on the wire between peer and transactor - if you need a vector, you have to make it a vector after the wire The encoding between peer and transactor doesn’t preserve Clojure vectors, which is why your example works using `with` (no wire) but doesn’t with a remote transactor (the vector becomes an array list). When you pass in `?path` in your query, it is bound to `?p` in your rule, where the call to `vector` evaluates true, when the passed in path is an array list or a vector. You would need to use an intermediate variable that you force into a new vector if you need the bottom of the rule to return a vector. I have provided a very small reproduction below (using an explicit array list): ```auto (def uri "datomic:mem://testdb") (d/create-database uri) (def conn (d/connect uri)) (def a-list (java.util.ArrayList.)) (.add a-list 2) (.add a-list 3) a-list ;; [2 3] (= a-list (vector 2 3)) ;; true (d/q '[:find ?a . :in $ ?a-list :where [(vector ?a-list) ?a]] (d/db conn) a-list) ;; [[2 3]] (type (first *1)) ;; java.util.ArrayList (d/q '[:find ?a . :in $ ?a-list :where [(vector ?b) ?a] [(into [] ?a-list) ?b]] (d/db conn) a-list) ;; [[2 3]] (type (first *1)) ;; clojure.lang.PersistentVector ```