I have possibly found a bug in the Validator component when combining these two constraints: Expression and All. I'm going to show it on an example:

Considering these two classes:

namespace AppBundle;
class ParentClass {

	private $name;
	private $children;

	public function __construct(){
		$this->children = new ArrayCollection();
	}

	public function getName() {
		return $this->name;
	}

	public function setName( $name ) {
		$this->name = $name;
	}

	public function getChildren() {
		return $this->children;
	}

	public function addChild( $child ) {
		$this->children->add($child);
	}
}

and

namespace AppBundle;
class ChildClass {

	private $name;

	public function getName() {
		return $this->name;
	}

	public function setName( $name ) {
		$this->name = $name;
	}

}

With the following validation rules stored in validation.yml:

AppBundle\ParentClass:
    properties:
        children:
            - NotBlank: ~
            - Valid: ~
            - All:
                - Expression:
                    expression: "this.getName() != value.getName()"

AppBundle\ChildClass:
    properties:
        name:
            - NotBlank: ~

I would like to achieve to forbid assigning the parent's name to children. So the getName() method of the parent should return a distinct value from getName() methods of all children.

However after trying to validate an instance of ParentClass constructed like this:

    $validator = $this->get('validator');

    $parent = new ParentClass();
    $parent->setName("foo");

    $child = new ChildClass();
    $child->setName("bar");

    $parent->addChild($child);
    $validator->validate($parent);

A RuntimeException is thrown with the following message:

Unable to get a property on a non-object. 

As I figured out, the variable value is correctly assigned to child objects, however the variable this in the expression has a value of null. I would expect a reference to the parent object in this variable, regarding to the documentation:

this: The object being validated (e.g. an instance of BlogPost);

Symfony 3.1.6, PHP 7.0.8, Linux Mint 18