Example of a OneToOne, bidirectional, PK as FK association.

oneToOneBidirectionalPkAsFK

<?php

class Book
{
    /**
     * @ORM\Id
     * @ORM\Column()
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    protected $id;

    /**
     * @ORM\OneToOne(targetEntity="HardCover", mappedBy="book", cascade={"persist"})
     */
    protected $hardCover;
    
    public function getHardCover()
    {
        return $this->hardCover;
    }
}

class HardCover
{
    /**
     * @ORM\Id
     * @ORM\OneToOne(targetEntity="Book", inversedBy="hardCover")
     * @ORM\JoinColumns({
     *     @ORM\JoinColumn(name="id_book", referencedColumnName="id")
     * })
     */
    protected $book;

    public function getBook()
    {
        return $this->book;
    }

    public function setBook(Book $book)
    {
        $this->book = $book;
    }
}