Doctrine - Integrity Constraint Violation logic - symfony

Question

I've been reviewing the Integrity constraint violation issues here on Stackoverflow for the past few days trying to resolve this issue, but I'm not having any success deducing the proper doctrine logic.
I have two entities that express a one-to-many and one-to-many relationship, but when I try to add and persist an item I get a insert error:
(in controller)
$user = $this->get('security.token_storage')->getToken()->getUser();
$chartnew = $Charts->setChartNo(9999);
$user->addUserschart($chartnew);
$em->persist($user);
$em->flush();
an exception occurred while executing 'INSERT INTO charts (chart_no, id_user) VALUES (?, ?)' with params [9999, null]:
SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'id_user' cannot be null
class Users implements UserInterface
{
/**
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Charts",cascade={"persist"}, mappedBy="charts")
*/
private $userscharts;
public function __construct()
{
$this->userscharts = new ArrayCollection();
//parent::__construct();
}
/**
* Add userschart
*
* #param \AppBundle\Entity\Charts $userschart
*
* #return Users
*/
public function addUserschart(\AppBundle\Entity\Charts $userschart)
{
$this->userscharts->add($userschart);
return $this;
}
.......
}
class Charts
{
/**
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\Users", inversedBy="userscharts")
* #ORM\JoinColumn(name="id_user", referencedColumnName="id_user")
*/
private $charts;
/**
* #var integer
*
* #ORM\Column(name="chart_no", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="NONE")
*/
private $chartNo;
/**
* #var integer
*
* #ORM\Column(name="id_user", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="NONE")
*/
private $idUser;
/**
* Set chartNo
*
* #param integer $chartNo
*
* #return Charts
*/
public function setChartNo($chartNo)
{
$this->chartNo = $chartNo;
return $this;
}
/**
* Get chartNo
*
* #return integer
*/
public function getChartNo()
{
return $this->chartNo;
}
/**
* Set idUser
*
* #param integer $idUser
*
* #return Charts
*/
public function setIdUser($idUser)
{
$this->idUser = $idUser;
return $this;
}
/**
* Get idUser
*
* #return integer
*/
public function getIdUser()
{
return $this->idUser;
}
/**
* Set charts
*
* #param \AppBundle\Entity\Users $charts
*
* #return Charts
*/
public function setCharts(\AppBundle\Entity\Users $charts = null)
{
$this->charts = $charts;
return $this;
}
/**
* Get charts
*
* #return \AppBundle\Entity\Users
*/
public function getCharts()
{
return $this->charts;
}
}