Company.php

<?php
//....
    /**
     * Many Companies has Many Countries
     * @ORM\ManyToMany(targetEntity="AppBundle\Entity\Country", inversedBy="companies")
     * @ORM\JoinTable(name="companies_countries")
     */
    private $countries;

CompanyType.php

<?php
//...
            ->add('countries', CollectionType::class, [
                'entry_type'   => CountryType::class,
                'allow_add' => true,
                'by_reference' => false,
                'allow_delete' => true,
                'required' => true,
            ])

Country.php

<?php

namespace AppBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

/**
 * Country
 *
 * @ORM\Table(name="country")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\CountryRepository")
 */
class Country
{
    /**
     * @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=50)
     */
    private $name;

    /**
     * @var string
     *
     * @ORM\Column(name="symbol", type="string", length=2)
     */
    private $symbol;


    /**
     * @var string
     *
     * @ORM\Column(name="slug", type="string", length=50)
     */
    private $slug;

    /**
     * @var string
     *
     * @ORM\Column(name="enabled", type="boolean")
     */
    private $enabled;

    /**
     * Many Countries have Many Companies.
     * @ORM\ManyToMany(targetEntity="AppBundle\Entity\Company", mappedBy="countries")
     */
    private $companies;

    /**
     * One Country has Many Voivodeships.
     * @ORM\OneToMany(targetEntity="AppBundle\Entity\Voivodeship", mappedBy="country")
     */
    private $voivodeships;


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

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

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

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

        return $this;
    }

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

    /**
     * @return string
     */
    public function getSymbol()
    {
        return $this->symbol;
    }

    /**
     * @param string $symbol
     */
    public function setSymbol($symbol)
    {
        $this->symbol = $symbol;
    }

    /**
     * @return string
     */
    public function getSlug()
    {
        return $this->slug;
    }

    /**
     * @param string $slug
     */
    public function setSlug($slug)
    {
        $this->slug = $slug;
    }

    /**
     * @return string
     */
    public function getEnabled()
    {
        return $this->enabled;
    }

    /**
     * @param string $enabled
     */
    public function setEnabled($enabled)
    {
        $this->enabled = $enabled;
    }
}

CountryType.php

<?php

namespace AppBundle\Form;

use AppBundle\Entity\Country;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class CountryType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('name', EntityType::class, [
                'class' => Country::class,
                'multiple' => true,
                'expanded' => true,
            ])
        ;
    }/**
     * {@inheritdoc}
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Country'
        ));
    }

    /**
     * {@inheritdoc}
     */
    public function getBlockPrefix()
    {
        return 'appbundle_country';
    }


}

County.php

<?php

namespace AppBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

/**
 * County
 *
 * @ORM\Table(name="county")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\CountyRepository")
 */
class County
{
    /**
     * @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=50)
     */
    private $name;

    /**
     * @var int
     *
     * @ORM\ManyToOne(targetEntity="AppBundle\Entity\Voivodeship", inversedBy="counties")
     * @ORM\JoinColumn(name="voivodeship", referencedColumnName="id")
     */
    private $voivodeship;

    /**
     * Many Groups have Many Users.
     * @ORM\ManyToMany(targetEntity="AppBundle\Entity\County", mappedBy="counties")
     */
    private $companies;


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

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

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

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

        return $this;
    }

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

    /**
     * @return int
     */
    public function getVoivodeship()
    {
        return $this->voivodeship;
    }

    /**
     * @param int $voivodeship
     */
    public function setVoivodeship($voivodeship)
    {
        $this->voivodeship = $voivodeship;
    }
}

Voivodeship.php

<?php

namespace AppBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

/**
 * Voivodeship
 *
 * @ORM\Table(name="voivodeship")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\VoivodeshipRepository")
 */
class Voivodeship
{
    /**
     * @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=50)
     */
    private $name;

    /**
     * @var string
     *
     * @ORM\Column(name="slug", type="string", length=50)
     */
    private $slug;

    /**
     * Many Features have One Product.
     * @ORM\ManyToOne(targetEntity="AppBundle\Entity\Country", inversedBy="voivodeships")
     * @ORM\JoinColumn(name="country", referencedColumnName="id")
     */
    private $country;

    /**
     * One Voivodeship has Many Counties.
     * @ORM\OneToMany(targetEntity="AppBundle\Entity\County", mappedBy="voivodeship", fetch="EAGER")
     */
    private $counties;


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

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

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

        return $this;
    }

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

    /**
     * @return string
     */
    public function getSlug()
    {
        return $this->slug;
    }

    /**
     * @param string $slug
     */
    public function setSlug($slug)
    {
        $this->slug = $slug;
    }

    /**
     * Set country
     *
     * @param integer $country
     *
     * @return Voivodeship
     */
    public function setCountry($country)
    {
        $this->country = $country;

        return $this;
    }

    /**
     * Get country
     *
     * @return int
     */
    public function getCountry()
    {
        return $this->country;
    }

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