bug: how to prevent ORM save/create to break ajax request?

Could you please provide the code that makes save/create to break an ajax request?

example: js:

function addListeArticleRequest() {
    var paramRaw = {};
    paramRaw['tarifClient'] = parseInt($("#tarif-c").text());
    paramRaw['selArticleCode'] = selectedArticleCode;
    var paramJson = JSON.stringify(paramRaw);
    $.ajax({
        type: "POST",
        dataType: 'json',
        data: {
            param: paramJson
        },
        headers: {'HTTP_X_REQUESTED_WITH': 'XMLHttpRequest'},
        url: 'localhost/appli/devis/newListArticle/',
        complete: function (response) {
            var rep = JSON.parse(response.responseText);
            var testResponse = rep.content;
            console.log(testResponse);
    });
}

php (controller):

        public function newListArticleAction()
        {
        if ($this->request->isPost()) {
            if ($this->request->isAjax()) {
                $receivedDatas = json_decode($this->request->getPost('param'));
                $newSelArticleCode = $receivedDatas->selArticleCode;
                $tarif = $receivedDatas->tarifClient;
                if (($tarif == '') || (!$tarif)) {
                    $tarif = 1;
                }
                $tarifs = array();
                $selectedArticle = Articles::findFirstByCode($newSelArticleCode);
                if (!$selectedArticle) {
                    $this->view->messagesInfo = "erreur: article introuvable";
                    $this->jsonMessages['contenu'] = 'erreur';
                } else {
                    $tarifs[0] = $selectedArticle->prixVente;
                    $tarifs[1] = $selectedArticle->prixLocation1;
                    $tarifs[2] = $selectedArticle->prixLocation2;
                   if( strcmp($receivedDatas->vl, "VENTE") == 0 ) {
                        $selectedArticle->tarif = $tarifs[0];
                    }
                    else {
                        $selectedArticle->tarif = $tarifs[$tarif];
                    }
                    $tarif = $selectedArticle->tarif;
                    $newListeArticle = new Listearticle();
                    $newListeArticle->tarifFinal = $selectedArticle->tarif;
                    $newListeArticle->articleID = $selectedArticle->id;
                    $newListeArticle->descriptionV = $selectedArticle->description;
                    $newListeArticle->designationV = $selectedArticle->designation;
                    $newListeArticle->stockVente = 0;  //TODO

                    $newListeArticle->save();

                    $this->jsonMessages['content'] = "test message";
                    return $this->jsonMessages;
                }
            }
        }
    }

so, here $newListeArticle object is correctly saved in DB (postgres), but response.responseText is empty. If I comment "$newListeArticle->save();" , then I get a response as expected

($this->jsonMessages is handled in ControllerBase )