Strange duplication in vector of strings when using the context to store values between handlers

[Erik Bakstad]

Hi. I noticed something strange when storing data in the context re-using the same key. I lookup a value in :allowed? and store it in the ::instance key of the context. If it's a PUT I assoc the result of the update to ::instance and return it in :handle-ok. For some strange reason, this causes a duplication in the vector :children? 

Here the boild down version of our code:

(defresource put-delete-get [system path id json-body {:keys [find-by-id find-versions

                                                              create update delete-by-id validation-fn

                                                              has-permission] :or {has-permission (constantly true)}}]

  :allowed-methods [:put :delete :get]

  :available-media-types ["application/json"]

  :allowed? (fn [{{method :request-method} :request}]

                  {::instance (find-by-id system id)})

  :exists? ::instance

  :can-put-to-missing? false

  :new? false

  :respond-with-entity? (fn [{{method :request-method} :request}] (= :put method))

  :put! (fn [ctx]

          (let [old (get ctx ::instance)

                new (update system json-body)]

            (println "###### Old ######\n")

            (clojure.pprint/pprint old)

            (println "###### New ######\n")

            (clojure.pprint/pprint new)

            (assoc ctx ::instance new)))

...

  :handle-ok (fn [ctx]

               (println "####### Result ######")

               (clojure.pprint/pprint (get ctx ::instance))

               (get ctx ::instance)))

=>

###### Old ######

{:children ["51e7e7ab30043ff5187d568e"],

 :_version 2}

###### New ######

{:children ["51e7e7ab30043ff5187d568e" "51e7e7b630043ff5187d5690"],

 :_version 3}

###### Result ######

{:children

 ["51e7e7ab30043ff5187d568e"

  "51e7e7ab30043ff5187d568e" <-- Duplicate!?

  "51e7e7b630043ff5187d5690"],

 :_version 3}

Is this a bug, or am I doing something stupid in my handlers?

Thanks

-Erik

[Erik Bakstad]

I forgot to mention that I don't have the same problem if I assign the result in put! to a new key and return that new key in :handle-ok.

[Philipp Meier]

Hi Erik,

I think you found a bug in the way liberator merges the return value of a decision. Liberator uses the combine function 

that is defined at https://github.com/clojure-liberator/liberator/blob/master/src/liberator/core.clj#L70 This sould enable

a developer to return "{::foo 1}" and liberator updates the context to become {:request {...} :resource {...} :representation {...} ::foo 1}

As you can see a list or vector value will be concatenated to the existing value. I cannot see why this is a sensible

implementation. 

Please fill a bug report asI cannot see a quick work around for you. I suggest that the deep combine logic is replaced with

a top level map merge. As a fallback we can allow decision function to return an function which is applied to the context:

(resource foo 

  :exists? (fn [ctx] (assoc ctx ::foo "bar")) 

  ;; this would not work

  :malformed? (fn [ctx] (dissoc ctx ::foo)))

  ;; this would remove the key

  :malformed? (fn [_] #(dissoc % ::foo)))

On the other hand there is a proposal to allow decision function to return function that will be used

as the next decision in the flow. What I sketched above would collide with this.

-billy.

[Erik Bakstad]

Hi Philipp.

Thanks for looking into it. Create issue: https://github.com/clojure-liberator/liberator/issues/60

Workaround is simply not using the same key, but IMHO it makes sense to keep the semantics of merge.

Cheers

Erik

[Philipp Meier]

Am Donnerstag, 18. Juli 2013 15:31:00 UTC+2 schrieb Erik Bakstad:

Hi. I noticed something strange when storing data in the context re-using the same key. I lookup a value in :allowed? and store it in the ::instance key of the context. If it's a PUT I assoc the result of the update to ::instance and return it in :handle-ok. For some strange reason, this causes a duplication in the vector :children? 

  :put! (fn [ctx]

          (let [old (get ctx ::instance)

                new (update system json-body)]

            (println "###### Old ######\n")

            (clojure.pprint/pprint old)

            (println "###### New ######\n")

            (clojure.pprint/pprint new)

            (assoc ctx ::instance new)))

I think the easies fix is  to use

(fn [ctx]

  ...

  {::instance ^:replace new})

In general avoid returning the full (update) context, especially if you have vectors it it. I'm still thinking about a way to change this without creating regessions in other places.

Philipp Meier

[Chris Hapgood]

I'm pleased to see this project still getting the author's attention.  We use liberator and love it.