building a trigger function to process json data with postgres

I'm struggling while building a pl/pgsql function to process a json array.

The data I receive from the client is a list of answers to questions by a user that I store in a json type field called 'data' in my table experiences.

[
{"completed":false},
{"completed":false},
{"completed":false},
{"completed":true,"score":1,"answerList":["1"],"QuestionId":75,"startTime":"2016-01-09T16:40:06.153Z","clickNb":1,"endTime":"2016-01-09T16:40:07.844Z"},
{"completed":true,"score":1,"answerList":["1"],"QuestionId":76,"startTime":"2016-01-09T16:40:08.487Z","clickNb":1,"endTime":"2016-01-09T16:40:12.482Z"},
{"completed":true,"score":1,"answerList":["1"],"QuestionId":77,"startTime":"2016-01-09T16:40:13.042Z","clickNb":1,"endTime":"2016-01-09T16:40:17.689Z"}
]

I want to build a trigger function to add up all the scores and save the result in a field called accuracy_score. This is what I have build so far:

CREATE OR REPLACE FUNCTION calculate_accuracy() RETURNS TRIGGER AS $$
DECLARE 
    dataarray json;

BEGIN
    dataarray := json_array_elements(New.data);
    UPDATE experiences SET accuracy_score = subquery.score 
    FROM (SELECT SUM(dataarray->>'score') from dataarray) AS subquery 
    WHERE challenge_id = New.challenge_id;

    RETURN NULL;
END;
$$ LANGUAGE plpgsql;


CREATE TRIGGER calculate_accuracy
AFTER INSERT ON experiences
FOR EACH ROW EXECUTE PROCEDURE calculate_accuracy();

I get the following error message when I try to run it:

ERROR: query "SELECT json_array_elements(New.data)" returned more than one row CONTEXT: PL/pgSQL function calculate_accuracy() line 6 at assignment

I don't find how to fix this. Help more than welcome on this!

If you are going to change a value in an inserted row, use BEFORE TRIGGER and change the record NEW in a trigger function:

CREATE OR REPLACE FUNCTION calculate_accuracy() RETURNS TRIGGER AS $$
BEGIN
    NEW.accuracy_score = (
        SELECT sum((e->>'score')::int)
        FROM json_array_elements(NEW.data) e);
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER calculate_accuracy
BEFORE INSERT ON experiences
FOR EACH ROW EXECUTE PROCEDURE calculate_accuracy();

Test:

insert into experiences (challange_id, data)
values
(1, 
'[
{"completed":false},
{"completed":false},
{"completed":false},
{"completed":true,"score":1,"answerList":["1"],"QuestionId":75,"startTime":"2016-01-09T16:40:06.153Z","clickNb":1,"endTime":"2016-01-09T16:40:07.844Z"},
{"completed":true,"score":1,"answerList":["1"],"QuestionId":76,"startTime":"2016-01-09T16:40:08.487Z","clickNb":1,"endTime":"2016-01-09T16:40:12.482Z"},
{"completed":true,"score":1,"answerList":["1"],"QuestionId":77,"startTime":"2016-01-09T16:40:13.042Z","clickNb":1,"endTime":"2016-01-09T16:40:17.689Z"}
]'::json)
returning challange_id, accuracy_score;

 challange_id | accuracy_score 
--------------+----------------
            1 |              3
(1 row)