CategoryAdmin
 protected function configureFormFields(FormMapper $form)
    {
        $form
            ->add('name',TextType::class)
            ->add('products',EntityType::class,
                [
                    'class' => Product::class,
                    'property' => 'name',
                    'required' => false,
                ]);
    }


<?php

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * Category
 *
 * @ORM\Table(name="category")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\CategoryRepository")
 */
class Category
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255, unique=true)
     */
    private $name;


    /**
     * @ORM\OneToMany(targetEntity="Product", mappedBy="category")
     */
    private $products;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="createdAt", type="datetime")
     */
    private $createdAt;

    /**
     * @var string
     *
     * @ORM\Column(name="updatedAt", type="string", length=255)
     */
    private $updatedAt;

    public function __toString()
    {
        return $this->name;
    }

    /**
     * @ORM\PrePersist()
     */
    public function setCreatedDate()
    {

        $this->createdAt = new \DateTime();
        $this->updatedAt = new \DateTime();
    }

    /**
     * @ORM\PreUpdate()
     */
    public function setUpdateDate()
    {
        $this->updatedAt = new \DateTime();
    }
    /**
     * Constructor
     */
    public function __construct()
    {
        $this->products = new \Doctrine\Common\Collections\ArrayCollection();
    }

    /**
     * Get id
     *
     * @return integer
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set name
     *
     * @param string $name
     *
     * @return Category
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get name
     *
     * @return string
     */
    public function getName()
    {
        return $this->name;
    }

    /**
     * Set createdAt
     *
     * @param \DateTime $createdAt
     *
     * @return Category
     */
    public function setCreatedAt($createdAt)
    {
        $this->createdAt = $createdAt;

        return $this;
    }

    /**
     * Get createdAt
     *
     * @return \DateTime
     */
    public function getCreatedAt()
    {
        return $this->createdAt;
    }

    /**
     * Set updatedAt
     *
     * @param string $updatedAt
     *
     * @return Category
     */
    public function setUpdatedAt($updatedAt)
    {
        $this->updatedAt = $updatedAt;

        return $this;
    }

    /**
     * Get updatedAt
     *
     * @return string
     */
    public function getUpdatedAt()
    {
        return $this->updatedAt;
    }


    /**
     * Add product
     *
     * @param \AppBundle\Entity\Product $product
     *
     * @return Category
     */
    public function addProduct(\AppBundle\Entity\Product $product = null)
    {
        $this->products[] = $product;

        return $this;
    }

    /**
     * Remove product
     *
     * @param \AppBundle\Entity\Product $product
     */
    public function removeProduct(\AppBundle\Entity\Product $product)
    {
        $this->products->removeElement($product);
    }

    /**
     * Get products
     *
     * @return \Doctrine\Common\Collections\Collection
     */
    public function getProducts()
    {
        return $this->products;
    }
}









<?php

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * Product
 *
 * @ORM\Table(name="product")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\ProductRepository")
 * @ORM\HasLifecycleCallbacks()
 */
class Product
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255, unique=true)
     */
    private $name;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="createdAt", type="datetime")
     */
    private $createdAt;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="updatedAt", type="datetime")
     */
    private $updatedAt;

    /**
     * @ORM\ManyToOne(targetEntity="Category", inversedBy="products")
     * @ORM\JoinColumn(name="category_id", referencedColumnName="id", nullable=true)
     */
    private $category;

    /**
     * THIS ALLOW TO SHOW PRETTY NAME
     *      Item "IPHONE" has been successfully created.
     * INSTEAD OF :
     *      Item "AppBundle\Entity\Product:00000000386cfe5f00000000bdb77d26" has been successfully created.
     * @return string
     */
    public function __toString()
    {
        return $this->name;
    }

    /**
     * @ORM\PrePersist()
     */
    public function setCreatedDate()
    {
        $this->createdAt = new \DateTime();
        $this->updatedAt = new \DateTime();
    }

    /**
     * @ORM\PreUpdate()
     */
    public function setUpdateDate()
    {
        $this->updatedAt = new \DateTime();
    }


    /**
     * Get id
     *
     * @return integer
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set name
     *
     * @param string $name
     *
     * @return Product
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get name
     *
     * @return string
     */
    public function getName()
    {
        return $this->name;
    }

    /**
     * Set createdAt
     *
     * @param \DateTime $createdAt
     *
     * @return Product
     */
    public function setCreatedAt($createdAt)
    {
        $this->createdAt = $createdAt;

        return $this;
    }

    /**
     * Get createdAt
     *
     * @return \DateTime
     */
    public function getCreatedAt()
    {
        return $this->createdAt;
    }

    /**
     * Set updatedAt
     *
     * @param \DateTime $updatedAt
     *
     * @return Product
     */
    public function setUpdatedAt($updatedAt)
    {
        $this->updatedAt = $updatedAt;

        return $this;
    }

    /**
     * Get updatedAt
     *
     * @return \DateTime
     */
    public function getUpdatedAt()
    {
        return $this->updatedAt;
    }


    /**
     * Set category
     *
     * @param \AppBundle\Entity\Category $category
     *
     * @return Product
     */
    public function setCategory(\AppBundle\Entity\Category $category = null)
    {
        $this->category = $category;

        return $this;
    }

    /**
     * Get category
     *
     * @return \AppBundle\Entity\Category
     */
    public function getCategory()
    {
        return $this->category;
    }
}