gistfile1.txt

//answer entity class

    /**
     * Set answer
     *
     * If answer is an array, converts to
     * string with 'CSV:' prepended. This
     * value is used to detect whether or not
     * to convert answer back to array for form
     * choice widget (checkbox requires array)
     *
     * @param string $answer
     * @return Answer
     */
    public function setAnswer($answer)
    {

        if(is_array($answer)){
            $answer = implode (',', $answer);
            $answer = 'CSV:'.$answer;
        }
        $this->answer = $answer;
    
        return $this;
    }

    /**
     * Set answer as array
     * 
     * Converts string answer into array
     *
     * @param $answer
     * @return $this
     */
    public function setAnswerAsArray($answer)
    {
        $answer = str_replace('CSV:', '', $answer);
        $this->answer = explode(',', $answer);

        return $this;
    }

    /**
     * Get answer
     *
     * @return string 
     */
    public function getAnswer()
    {
        return $this->answer;
    }

gistfile2.txt

// event listener that sets data in form before it is displayed

     if(substr($answer->getAnswer(), 0, 4) == "CSV:"){
         $answer->setAnswerAsArray($answer);
     }