Optional ManyToOne relationship in Doctrine 2 - php

Question

I'm building an application for SNMP Trap logging. It has two MySQL tables: one with the traps, and one with the hosts that I want to log.
The traps table is filled externally. The hosts can be entered through the website. A host CAN have one or many traps. A trap CAN have one host.
So this is a many to one relationship, but optional on both sides. How do I implement this, since Doctrine requires for one of the members to be a primary key, and therefore not nullable?
Code for both classes:
class Trap
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="eventname", type="string", length=50)
*/
private $eventname;
/**
* #var string
*
* #ORM\Column(name="eventid", type="string", length=50)
*/
private $eventid;
/**
* #var string
*
* #ORM\Column(name="trapoid", type="string", length=100)
*/
private $trapoid;
/**
* #var string
*
* #ORM\Column(name="enterprise", type="string", length=100)
*/
private $enterprise;
/**
* #var string
*
* #ORM\Column(name="community", type="string", length=20)
*/
private $community;
/**
* #var string
*
* #ORM\Column(name="hostname", type="string", length=255)
*/
private $hostname;
/**
* #var string
*
* #ORM\Column(name="agentip", type="string", length=16)
*/
private $agentip;
/**
* #var string
*
* #ORM\Column(name="category", type="string", length=20)
*/
private $category;
/**
* #var string
*
* #ORM\Column(name="severity", type="string", length=255)
*/
private $severity;
/**
* #var string
*
* #ORM\Column(name="uptime", type="string", length=20)
*/
private $uptime;
/**
* #var datetime
*
* #ORM\Column(name="traptime", type="datetime")
*/
private $traptime;
/**
* #var string
*
* #ORM\Column(name="formatline", type="string", length=255)
*/
private $formatline;
/**
* #ORM\ManyToOne(targetEntity="Host", inversedBy="traps")
* #ORM\JoinColumn(name="agentip", referencedColumnName="ip", nullable=true)
*/
protected $host;
}
class Host
{
/**
* #var string
* #ORM\Id
* #ORM\Column(name="ip", type="string", length=16)
*/
private $ip;
/**
* #ORM\Column(name="hostname", type="string", length=100)
*/
private $hostname;
/**
* #ORM\Column(name="type", type="string", length=100)
*/
private $type;
/**
* #ORM\Column(name="importance", type="integer", length=1)
*/
private $importance;
/**
* #ORM\OneToMany(targetEntity="Trap", mappedBy="host")
*/
protected $traps;
}
Edited with error
The error is that I'm not allowed to delete or truncate my hosts table, which should be possible in my application, because traps must be able to exist without a host.
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`snmptt`.`snmptt`, CONSTRAINT `FK_9BF059B998B5BE9E` FOREIGN KEY (`agentip`) REFERENCES `hosts` (`ip`))

Answer 1

Posted as answer from comment as believe to be appropriate answer;
"If you are trying to remove this as straight sequel then yes the error is correct you cannot remove the parent before unlink the child, if there is no parent then this is fine if so unlink first then remove"