__toString return name from another entity - symfony

Question

Let say I have 2 tables in my database : lesson and group. Group can have 1 lesson per year.
Then I will have 2 entities, lesson entity :
<?php
namespace Sifo\AdminBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
class Lesson
{
/**
* #var integer
*/
private $id;
/**
* #var string
*/
private $name;
/**
* Set id
*
* #param integer $id
* #return DftGrupMapel
*/
public function setId($id)
{
$this->id = $id;
return $this;
}
/**
* Get id
*
* #return integer
*/
public function getId()
{
return $this->id;
}
and group entity :
<?php
namespace Sifo\AdminBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
class Group
{
/**
* #var integer
*/
private $id;
/**
* #var \Sifo\AdminBundle\Entity\MstLesson
*/
private $idLesson;
/**
* #var integer
*/
private $year;
/**
* Set id
*
* #param integer $id
* #return DftGrup
*/
public function setId($id)
{
$this->id = $id;
return $this;
}
/**
* Get id
*
* #return integer
*/
public function getId()
{
return $this->id;
}
/**
* Set idLesson
*
* #param \Sifo\AdminBundle\Entity\MstLesson $idLesson
* #return DftGrup
*/
public function setIdLesson(\Sifo\AdminBundle\Entity\MstLesson $idLesson = null)
{
$this->idLesson = $idLesson;
return $this;
}
/**
* Get idGrup
*
* #return \Sifo\AdminBundle\Entity\MstGrup
*/
public function getIdLesson()
{
return $this->idLesson;
}
/**
* Set year
*
* #param integer $year
* #return DftGrup
*/
public function setYear($year)
{
$this->year = $year;
return $this;
}
/**
* Get year
*
* #return integer
*/
public function getYear()
{
return $this->year;
}
The problem is, in group entity I don't have available string to show for __toString(). I fell not good for making new name field in group entity and save lesson name there.
lesson.id and group.idLesson is foreign key.
How I can get name field in lesson entity to show in __toString() group without create new field?

Answer 1

This is an ORM. Your Group class shouldn't really care about what it's associated Lesson's id is... it should just have a reference to the Lesson object itself:
<?php
namespace Sifo\AdminBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
class Group
{
private $id; // An integer
private $lesson; // A Lesson object
private $year; // An integer, probably
Once $lesson is a Lesson object, you can refer to it in your Group::__toString():
private function __toString() {
return $this->lesson->getName();
}
Here's the documentation on association mapping, which should be helpful in mapping the relationship between Group and Lesson:
http://docs.doctrine-project.org/en/2.0.x/reference/association-mapping.html

Answer 2

public function setIdLesson(\Sifo\AdminBundle\Entity\MstLesson $idLesson = null)
{
$this->idLesson = $idLesson;
return $this;
}
Here,you don't need the return statement.