How to correctly pass json and get it in the controller?

Question

AJAX

Pass json array to controller (namesDp)

$http({
            method: "POST",
            url: "http://localhost/cabinet/contracts/addContract",
            data: $.param({
                ......
                namesDp: $scope.data.model
            }),

In the controller I do this Well, I pass this array to the model

$dpList = json_decode($_POST['namesDp']);

if($this->model->addContract($dpList, $contractNumber,$contractTitle, $contractDate . . . ..

Here is the model

public function addContract( $dpList, $contractNumber,$contractTitle, $contractDate . . . . .. . . ) {


    foreach($dpList as $value) {

      $sql = "INSERT INTO contract(department, numberContract, nameContract, dateConclusion)
    VALUES(:department, :numberContract, :nameContract, :dateConclusion)
    ";
      $stmt = $this->db->prepare($sql);
      $stmt->bindValue(":department", $value, PDO::PARAM_INT);
      $stmt->bindValue(":numberContract", $contractNumber, PDO::PARAM_INT);
      $stmt->bindValue(":nameContract", $contractTitle, PDO::PARAM_STR);
      $stmt->bindValue(":dateConclusion", $contractDate, PDO::PARAM_STR);
      $stmt->execute();

. . ... .

There is an error
json_decode () expects parameter 1 to be string, array given in

Perhaps there are more errors .. when iterating over an array