symfony2 一对多，双向
例子
这种双向映射需要mappedBy的属性，OneToMany协会和inversedBy该属性ManyToOne关联。
双向关系既有拥有的一面，也有相反的一面。OneToMany关系可以使用连接表，因此您必须指定拥有方。该OneToMany协会始终是一个双向关联的另一方。
username = $username;
$this->group = $group;
}
/**
* Set username
*
* @param string $username
*/
public function setUsername($username)
{
$this->username = $username;
}
/**
* Get username
*
* @return string
*/
public function getUsername()
{
return $this->username;
}
/**
* @param Group|null $group
*/
public function setGroup(Group $group = null)
{
if($this->group !== null) {
$this->group->removeUser($this);
}
if ($group !== null) {
$group->addUser($this);
}
$this->group = $group;
}
/**
* Get group
*
* @return Group|null
*/
public function getGroup()
{
return $this->group;
}
}name = $name;
$this->users = new ArrayCollection();
}
/**
* @return string
*/
public function getName()
{
return $this->name;
}
/**
* @param string $name
*/
public function setName($name)
{
$this->name = $name;
}
public function addUser(User $user)
{
if (!$this->getUsers()->contains($user)) {
$this->getUsers()->add($user);
}
}
public function removeUser(User $user)
{
if ($this->getUsers()->contains($user)) {
$this->getUsers()->removeElement($user);
}
}
public function getUsers()
{
return $this->users;
}
public function __toString()
{
return (string) $this->getName();
}
}