如何使用Doctrine在Symfony 4中组合两个独特的列？

Question

I have a table named 'student_assignment' in which I have multiple columns from which I am showing 2 of them below:

Both of these columns are also foreign keys.

StudentId   assignmentId
    10           7           -> allowed
    10           8           -> allowed
    11           7           -> allowed
    11           7           -> not allowed, the combination of 11 7 already exists in table

I have tried this in my entity file, but it does not work.

/**
 * Webkul\CampusConnect\Entity\StudentAssignment
 *
 * @Table(name="student_assignment", 
 *    uniqueConstraints={
 *        @UniqueConstraint(name="assignment_unique", 
 *            columns={"student", "assignment"})
 *    }
 * )
 * @Entity
 */

Please how to implement this using ORM in symfony 4.

I have a link which does ther same but in Mysql. I want the solution for Symfony ORM. enter link description here

Error:

[Semantical Error] The annotation "@Table" in class Webkul\CampusConnect\En tity\StudentAssignment was never imported. Did you maybe forget to add a "u se" statement for this annotation?

Entity:

namespace Webkul\CampusConnect\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\ORM\Mapping\Table;

/**
 * Webkul\CampusConnect\Entity\StudentAssignment
 *
 * @ORM\Table(name="student_assignment", 
 *    uniqueConstraints={
 *        @UniqueConstraint(name="assignment_unique", 
 *            columns={"student", "assignment"})
 *    }
 * )
 * @Entity
 */
class StudentAssignment
{
    /**
     * @ORM\Id()
     * @ORM\GeneratedValue()
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @ORM\ManyToOne(targetEntity="Webkul\CampusConnect\Entity\Student", inversedBy="studentAssignments")
     * @ORM\JoinColumn(onDelete="CASCADE")
     */
    private $student;