SonataAdminBundle forms One to Many or Many to Many does not persist

This problem occurs due to the `inverse-side` object is not defined in the `owning-side`. In order to solve this, we have to manually specify it in the `owning-side` class. Suppose that we have a user class and a role class that are `inverse-side` and `owning-side` respectively. Here is the example of defining the `inverse-side` object.

//Acme/Bundle/DemeBundle/Entity\User
public function addRole(\Enstb\Bundle\VisplatBundle\Entity\Role $role)
{
    // Link each role with the user 
    // (This is important for SonataAdminBundle!!)
    $role->addUser($this);
    $this->roles->add($role);
    return $this;
}
public function removeRole(\Enstb\Bundle\VisplatBundle\Entity\Role $role)
{
    // Link each role with the user
    $role->removeUser($this);
    $this->roles->removeElement($role);
}
public function getRoles()
{
    return $this->roles;
}

The most important part is `$role->addUser($this)`. It sends the object of `Users` class to be added in the `Roles` class. This makes the `SonataAdminBundle` to realize that which object should be linked together and persisted to a database.
And here is an example of `addUser` method in the `Roles` class:

//Acme/Bundle/DemeBundle/Entity\Role
public function addUser(\Enstb\Bundle\VisplatBundle\Entity\Users $user)
{
    $this->Users->add($user);
    return $this;
} 

Note: if you are not sure what is owning-side and inverse-side, read more about What is the difference between inversedBy and mappedBy?.