Working example of PHP Doctrine 2 ManyToOne with corresponding OneToMany association.

Bank.php

<?php

declare(strict_types=1);

namespace Banks\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Branches\Entity\Branch;

/**
 * https://www.doctrine-project.org/projects/doctrine-orm/en/2.6/reference/basic-mapping.html
 *
 * @ORM\Entity
 * @ORM\Table(name="banks")
 **/
class Bank
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer", name="id", nullable=false)
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    protected $id;

    /**
     * A Bank could have Many Branches
     *
     * @ORM\OneToMany(targetEntity="Branches\Entity\Branch", mappedBy="bank")
     *
     */
    protected $branches;

    /**
     * @ORM\Column(type="string", nullable=true)
     */
    protected $name;

    /**
     * @param bool $recursive
     * @return array|mixed
     */
    public function getBank($recursive=true)
    {
        return [
            'id' => $this->id,
            'name' => $this->name,
            'branches' => $this->withBranches($recursive),
        ];
    }
    
    /**
     * @param $recursive
     * @return mixed|array
     */
    public function withBranches($recursive)
    {
        if ($recursive) {
            return $this->getBranches()->map(function(Branch $branch) {
                // clears $branch->institution() to prevent infinite recursion
                $branch->resetBank();
                return $branch->getBranch();
            })->toArray();
        }

        return ['see_parent'];
    }

    /**
     * Institution constructor.
     */
    public function __construct()
    {
        $this->branches = new ArrayCollection();
    }

    /**
     * @return Collection
     */
    public function getBranches(): Collection
    {
        return $this->branches;
    }
use Branches\Entity\Branch;
    // ... Other getter/setters removed
}

Branch.php

<?php

declare(strict_types=1);

namespace Branches\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * https://www.doctrine-project.org/projects/doctrine-orm/en/2.6/reference/basic-mapping.html
 *
 * @ORM\Entity
 * @ORM\Table(name="branches")
 **/
class Branch
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer", nullable=false)
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    protected $id;

    /**
     * A Branch has one Bank
     *
     * @ORM\ManyToOne(targetEntity="Banks\Entity\Bank", inversedBy="branches")
     * @ORM\JoinColumn(name="bank_id", referencedColumnName="id")
     */
    protected $bank;

    /**
     * @ORM\Column(type="integer", nullable=false)
     */
    protected $bank_id;

    /**
     * @ORM\Column(type="string", nullable=true)
     */
    protected $name;

    /**
     * @param bool $recursive
     * @return array
     */
    public function getBranch($recursive=true)
    {
        return [
            'id' => $this->id,
            'bank_id' => $this->bank_id,
            'name' => $this->name,
            // getBank returns an object which json_encode can't handle
            'bank' => $this->getBank($recursive)
        ];
    }

    /**
     * @param $recursive
     * @return mixed|array
     */
    public function getBank($recursive)
    {
        if ($recursive)
        {
            return $this->bank;
        }

        return $this->bank->getBank(false);
    }
    
    public function setBank($bank)
    {
        $this->bank = $bank;
    }
    
    /**
     * Resets Bank Object in ManyToOne relationship to prevent infinite recursion
     *
     */
    public function resetBank(): void
    {
        $this->bank = ['see_parent'];
    }

    // ... Other getter/setters removed
}

BranchesCreateHandler.php

<?php

declare(strict_types=1);

namespace Branches\Handler;

use Psr\Container\ContainerInterface;
use Zend\Expressive\Helper\ServerUrlHelper;
use Doctrine\ORM\EntityManager;
use Doctrine\ORM\EntityRepository;
use Branches\Entity\Branch;
use Banks\Entity\Bank;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Zend\Diactoros\Response\JsonResponse;
use Zend\Diactoros\Response\RedirectResponse;

class BranchesCreateHandler implements RequestHandlerInterface
{
    protected $entityManager;
    protected $entityRepository;
    protected $entity;
    protected $urlHelper;
    protected $bank;

    /**
     *   the following are gained from the referring Factory, which gets them this way
     *
     *   $entityManager = $container->get(EntityManager::class);
     *   $urlHelper = $container->get(ServerUrlHelper::class);
     *   $entityManager->getConfiguration()->addEntityNamespace('Branch', 'Branches\Entity');
     *   $entityRepository = $entityManager->getRepository('Branch:Branch');
     *   $entity = new Branch();
     *   $bank = new Bank();
    */
    public function __construct(
        EntityManager $entityManager,
        EntityRepository $entityRepository,
        Branch $entity,
        ServerUrlHelper $urlHelper,
        Bank $bank,
    ) {
        $this->entityManager = $entityManager;
        $this->entityRepository = $entityRepository;
        $this->entity = $entity;
        $this->urlHelper = $urlHelper;
        $this->bank = $bank;
    }

    /**
     * {@inheritDoc}
     */
    public function handle(ServerRequestInterface $request) : ResponseInterface
    {
        $body = $request->getParsedBody()['Request']['Branches'];

        if (empty($body)) {
            // redirect to '/branches/' as a GET request
            return new RedirectResponse('/branches/');
        }
        
        $this->entity->setBank($bank);

        $this->entity->setBankId($body['bank_id']);
        $this->entity->setName($body['name']);
        $this->entity->setAddress1($body['address1']);
        $this->entity->setAddress2($body['address2']);
        $this->entity->setCity($body['city']);
        $this->entity->setZoneId($body['zone_id']);
        $this->entity->setZip($body['zip']);
        $this->entity->setEmail($body['email']);
        $this->entity->setPhone($body['phone']);
        $this->entity->setIsActive(1);
        $this->entity->setCreated(new \DateTime("now"));
        $this->entity->setModified(new \DateTime("now"));
      
      // if I print_r($this->entity) I see all fields just set above.

        try {
            $this->entityManager->persist($this->entity);
            $this->entityManager->flush();
          
// receiving error saying "Integrity constraint violation: 1048 Column 'bank_id' cannot be null"
            // if I remove the ManyToOne association on bank_id in Branch Entity, all works fine
          
        } catch(\Exception $e) {
            echo $e->getMessage();
        }

        $result['Result']['Branches'] = $this->entity->getBranch();

        if (empty($result['Result']['Branches'])) {
            echo "Not Created.\n";
            exit(1);
        }

        return new JsonResponse($result, 201);
    }
}

BranchesViewHandler.php

<?php

declare(strict_types=1);

namespace Branches\Handler;

use Zend\Expressive\Helper\ServerUrlHelper;
use Doctrine\ORM\EntityManager;
use Doctrine\ORM\EntityRepository;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Zend\Diactoros\Response\JsonResponse;

class BranchesViewHandler implements RequestHandlerInterface
{
    protected $entityManager;
    protected $entityRepository;
    protected $urlHelper;

    public function __construct(
        EntityManager $entityManager,
        EntityRepository $entityRepository
    ) {
        $this->entityManager = $entityManager;
        $this->entityRepository = $entityRepository;
    }

    /**
     * {@inheritDoc}
     */
    public function handle(ServerRequestInterface $request) : ResponseInterface
    {
        // returns entityRepository with Configuration and Repository already set for Entity
        $return = $this->entityRepository->find($request->getAttribute('id'));

        if ($return === null) {
            echo "Not found.\n";
            exit(1);
        }

        $result['Result']['Branch'] = $return->getBranch(false);

        return new JsonResponse($result);
    }
}