如何定义默认值学说实体？

I have two tables: plans and marks.

For this two tables

<?php
/**
 * @Entity
 * @Table(name="marks")
 */
class Mark{

    /**
     * @ManyToOne(targetEntity="Plan", inversedBy="marks")
     * @JoinColumn(name="plan_id", referencedColumnName="id")
      **/
    protected $plan = null;
    ...
}

And

<?php
/**
 * @Entity
 * @Table(name="plans")
 */
class Plan{

     /**
     * @OneToMany(targetEntity="Mark", mappedBy="plan")
     */
     private $marks;
     ...
}

As you can understand from this code in table marks I have field plan_id (which store plan id). This field is not mandatory and I want by default that plan_id = 0, but I can do it. If I set field $plan in class Mark = 0, I have error that was expected object. But If I set plan = 0, it save NULL in the database, I don't want it at all.

Maybe somebody has some advices?

Use lifecycle callbacks on Mark->setPlan()

/**
 * @Entity
 * @Table(name="marks")
 * @ORM\HasLifecycleCallbacks
 */
class Mark{

    /**
     * @ManyToOne(targetEntity="Plan", inversedBy="marks")
     * @JoinColumn(name="plan_id", referencedColumnName="id")
     **/
    protected $plan = null;

   /**
    * @ORM\PrePersist
    * @ORM\PreUpdate
    */    
   public function setPlan(Plan $plan = null){
      if(is_null($var)){
        $this->plan = 0;
      }
   }

But I think that Doctrine will return errors trying to find a plan with id = 0.