fix nested admin (one-to-one-to-many relation) that opens inline (fixes #6777)

[vv12131415]

Subject

Fix form rendering of nested Admin properties.

I am targeting this branch, because everything is backwards compatible.

Closes #6777.

Changelog

### Fixed
How `AdminHelper::appendFormFieldElement` renders nested forms.

[vv12131415]

tests will fail, because I have no idea how do you mock methods that does not exist on interface (talking about hasFormFieldDescription). yes, I've google it

[VincentLanglet]

tests will fail, because I have no idea how do you mock methods that does not exist on interface (talking about hasFormFieldDescription). yes, I've google it

$this->getMockBuilder(Interface::class)
            ->addMethods(['method'])
            ->getMockForAbstractClass();

?

[VincentLanglet]

Can you add a test where hasFormFieldDescription return false ?

[vv12131415]

There isnone case that I dont know how to handle.
There are nested if

            if ($collection instanceof DoctrinePersistentCollection || $collection instanceof PersistentCollection) {
                //since doctrine 2.4
                $modelClassName = $collection->getTypeClass()->getName();
            } elseif ($collection instanceof Collection) {
                $modelClassName = $this->getEntityClassName($admin, explode('.', preg_replace('#\[\d*?\]#', '', $path)));
            } else {
                throw new \Exception('unknown collection class');
            }

I can check for exception. But how should I handle case with PersistentCollection?
The short answer is - no, just don't check

[VincentLanglet]

I find the code

if ($collection instanceof DoctrinePersistentCollection || $collection instanceof PersistentCollection) {
                //since doctrine 2.4
                $modelClassName = $collection->getTypeClass()->getName();
            } elseif ($collection instanceof Collection) {
                $modelClassName = $this->getEntityClassName($admin, explode('.', preg_replace('#\[\d*?\]#', '', $path)));
            } else {
                throw new \Exception('unknown collection class');
            }

weird, since a PersistentCollection is a Collection, we should be able to remove the if part.

the collection is

$collection = $this->propertyAccessor->getValue($subject, $path);

So you could mock the propertyAccessor.

[VincentLanglet]

Rebasing 3.x should fix the build

[vv12131415]

some why on my local machine php-cs-fixer was trying to fix E_USER_DEPRECATED in opposite way

-        ), \E_USER_DEPRECATED);
+        ), E_USER_DEPRECATED);

I don't really understand why

[VincentLanglet]

Can you try composer update ? or a rebase ?

[vv12131415]

Can you try composer update ? or a rebase ?

1. deleted cache
2. rebased to latest commit on 3.x
3. composer update
   still the same.

Any way, I hope that it's been final fixes of this branch, other way I'll fix it some how

[phansys]

Could you please use the PR template in order to expose the motivation for these changes in the "Changelog" section?
Thank you.

[phansys]

If this PR is fixing something, the "Changelog" section should include the "Fixed" item.

[OskarStark]

Thank you