PersistentCollection::clear() modifies uow for DEFERRED_EXPLICIT change tracking policy

[paxal]

Bug Report

Q	A
BC Break	no
Version	2.6.3

Summary

Calling clear() on a Many-to-many association collection should not modify uow as long as persist() is not called on owner.
This is not the same behavior as calling removeElement() on each element, which sounds weird.

Current behavior

Calling clear() generates a DELETE FROM table WHERE owner_id = ? statement on a flush().

How to reproduce

Mapping :

<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping">
    <entity name="Foo"
            change-tracking-policy="DEFERRED_EXPLICIT">
        <id name="id" type="integer" column="id">
            <generator strategy="IDENTITY"/>
        </id>
        <many-to-many field="bars" target-entity="Bar">
            <cascade>
                <cascade-all/>
            </cascade>
            <join-table name="foo_bar">
                <join-columns>
                    <join-column name="foo_id" nullable="false"/>
                </join-columns>
                <inverse-join-columns>
                    <join-column name="bar_id" nullable="false"/>
                </inverse-join-columns>
            </join-table>
        </many-to-many>
    </entity>
    <entity name="Bar"
            change-tracking-policy="DEFERRED_EXPLICIT">
        <id name="id" type="integer" column="id">
            <generator strategy="IDENTITY"/>
        </id>
    </entity>
</doctrine-mapping>

with PHP code :

class Bar { public $id; }
class Foo {
    public $id;
    public $bars;
    public function __construct()
    {
        $this->bars = new ArrayCollection();
    }
}

// Create new entity
$foo = new Foo();
// Attach Bar
$foo->bars->add(new Bar());
// Persist the whole thing
$em->persist($foo);
$em->flush();
// Retrieve assigned id
$fooId = $foo->id;
// Clear em
$em->clear();

// Fetch from DB
$foo = $em->find(Foo::class, $fooId);
// Clear bars item per item
foreach ($foo->bars as $bar) {
    $foo->bars->removeElement($bar);
}
// Does nothing as $foo is not persisted
$em->flush();

$foo->bars->clear();
// Removes all elements even though $foo is not persisted
$em->flush();

Expected behavior

No statement should be executed.

[lcobucci]

Handled by #7761