<?php

class Category
{
    /**
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     */
    private $id;

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

    /**
     * @ORM\Column(type="boolean")
     */
    private $admin;

    /**
     * @ORM\OneToMany(targetEntity="App\Entity\Post", mappedBy="category")
     */
    private $posts;

    public function __construct()
    {
        $this->posts = new ArrayCollection();
    }
}

<?php

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity
 * @ORM\Table(name="posts")
 */
class Post
{
    /**
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     */
    private $id;

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

    /**
     * @ORM\Column(type="text")
     */
    private $content;

    /**
     * @ORM\ManyToOne(targetEntity="App\Entity\Category", inversedBy="posts")
     * @ORM\JoinColumn(nullable=false)
     */
    private $category;
}