Symfony2一对一的关系形式

我需要一些帮助才能理解我做错了什么,我试图在这里搜索一个答案,但没有任何效果.

在这里我的问题..

我有两个具有一对一关系的实体:

...
class FarmInfo
{
    ...

    /**
     * @var integer
     *
     * @ORM\Column(name="areaCovered", type="integer", nullable=true)
     */
    private $areaCovered;

    /**
     * @var integer
     *
     * @ORM\Column(name="totalIrrigatedArea", type="integer", nullable=true)
     */
    private $totalIrrigatedArea;

    /**
     * @var integer
     *
     * @ORM\Column(name="totalStaff", type="integer", nullable=true)
     */
    private $totalStaff;

    /**
     * @var DairyInfo
     * @ORM\OneToOne(targetEntity="DairyInfo", mappedBy="farmInfo", cascade={"persist","remove"})
     */
    private $dairyInfo;

    ...
}

和

...
class DairyInfo
{
    ...

    /**
     * @var FarmInfo
     *
     * @ORM\OneToOne(targetEntity="FarmInfo", inversedBy="dairyInfo", cascade={"persist"})
     * @ORM\JoinColumn(name="farmInfoId", referencedColumnName="id")
     * 
     */
    private $farmInfo;

    /**
     * @var integer
     *
     * @ORM\Column(name="peakCowMilked", type="integer", nullable=true)
     */
    private $peakCowMilked;

    /**
     * @var integer
     *
     * @ORM\Column(name="cowsMilkedInWinter", type="smallint", nullable=true)
     */
    private $cowsMilkedInWinter;

    ...
}

这里的formTypes:

class FarmInfoType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('areaCovered');
        $builder->add('totalIrrigatedArea');
        $builder->add('totalStaff');

        $builder->add('dairyInfo', new DairyInfoType());
    }

    /**
     * @param OptionsResolverInterface $resolver
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'FOO\CoreBundle\Entity\FarmInfo'
        ));
    }

    /**
     * @return string
     */
    public function getName()
    {
        return 'foo_corebundle_farminfo';
    }
}

class DairyInfoType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('peakCowMilked');
        $builder->add('cowsMilkedInWinter');
    }

    /**
     * @param OptionsResolverInterface $resolver
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'FOO\CoreBundle\Entity\DairyInfo'
        ));
    }

    /**
     * @return string
     */
    public function getName()
    {
        return 'foo_corebundle_dairyinfo';
    }
}

使用与我想要的格式相同的FarmInfo和DairyInfo为我创建表单,但是当我坚持使用时,FarmInfo仍然存在,DairyInfo会保留所有字段,但farmInfoId始终为null:

+---------------+--------------------+------------+
| peakCowMilked | cowsMilkedInWinter | farmInfoId |
+---------------+--------------------+------------+
|            11 |                 11 |       NULL |
+---------------+--------------------+------------+

我该怎么办symfony自动获取此ID？

谢谢大家的提前.