diff --git a/.github/workflows/static.yml b/.github/workflows/static.yml
index 007d37859..936eb3331 100644
--- a/.github/workflows/static.yml
+++ b/.github/workflows/static.yml
@@ -17,7 +17,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.1'
+          php-version: '8.3'
           extensions: "curl,dom,json,xml,dom"
           coverage: none
 
diff --git a/.github/workflows/test-application.yaml b/.github/workflows/test-application.yaml
index ed7107c6e..1a23e1c08 100644
--- a/.github/workflows/test-application.yaml
+++ b/.github/workflows/test-application.yaml
@@ -27,7 +27,6 @@ jobs:
 
             matrix:
                 php-version:
-                    - '8.0'
                     - '8.1'
                     - '8.2'
                     - '8.3'
@@ -35,12 +34,9 @@ jobs:
                 symfony-version: ['*']
                 jackalope-version: ['1.*']
                 include:
-                  - php-version: '8.0'
+                  - php-version: '8.1'
                     dependencies: lowest
                     symfony-version: '*'
-                  - php-version: '8.0'
-                    dependencies: highest
-                    symfony-version: '6.*'
                   - php-version: '8.1'
                     dependencies: highest
                     symfony-version: '5.4.*'
@@ -109,7 +105,6 @@ jobs:
 
             matrix:
                 php-version:
-                    - '8.0'
                     - '8.1'
                     - '8.2'
                     - '8.3'
@@ -117,12 +112,9 @@ jobs:
                 symfony-version: ['*']
                 jackalope-version: ['1.*']
                 include:
-                  - php-version: '8.0'
+                  - php-version: '8.1'
                     dependencies: lowest
                     symfony-version: '*'
-                  - php-version: '8.0'
-                    dependencies: highest
-                    symfony-version: '6.*'
                   - php-version: '8.1'
                     dependencies: highest
                     symfony-version: 5.4.*
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 85500d44e..0693a5be6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,16 @@ Changelog
 
 * Removed annotation mappings. Use attributes (or XML or YAML) instead.
 
+* `ChildrenCollection::slice` no longer accepts a node name as offset. If you
+  want a slice starting from a node name, use `ChildrenCollection::sliceByChildName`
+  instead.
+
+* Removed `DocumentManager::merge()`
+
+* Removed support for short namespace aliases. ``ClassMetadataFactory::getFqcnFromAlias`` and the namespace registering
+  on `Configuration` are removed. `ClassMetadataFactory` methods now require their `$className` argument to be an
+  actual FQCN.
+
 ### New Features
 
 * `DocumentManager::getDocumentId()` to get the id of a managed document
@@ -25,7 +35,7 @@ Changelog
 
 * Allow using `doctrine/persistence`
 
-* Added support for Symfony 7. Dropped support for PHP < 8.0 and Symfony < 5.4.
+* Added support for Symfony 7. Dropped support for PHP < 8.1 and Symfony < 5.4.
 
 1.x
 ===
diff --git a/composer.json b/composer.json
index e57c11a89..d7bcb2740 100644
--- a/composer.json
+++ b/composer.json
@@ -21,13 +21,13 @@
     "minimum-stability": "beta",
     "prefer-stable": true,
     "require": {
-        "php": "^8.0",
-        "doctrine/collections": "^1.0 || ^2.0",
+        "php": "^8.1",
+        "doctrine/collections": "^2.0",
         "doctrine/common": "^2.4 || ^3.0",
         "doctrine/annotations": "^1.14.3 || ^2.0",
         "doctrine/data-fixtures": "^1.0",
         "doctrine/event-manager": "^1.0 || ^2.0",
-        "doctrine/persistence": "^2.4",
+        "doctrine/persistence": "^3.0",
         "phpcr/phpcr": "^2.1.1",
         "phpcr/phpcr-implementation": "^2.1",
         "phpcr/phpcr-utils": "^1.3.0 || ^2.0",
diff --git a/docs/en/reference/association-mapping.rst b/docs/en/reference/association-mapping.rst
index 6585ef3c9..58ee3650c 100644
--- a/docs/en/reference/association-mapping.rst
+++ b/docs/en/reference/association-mapping.rst
@@ -518,6 +518,7 @@ collection of related documents. Say we have a User document that
 contains a collection of groups::
 
     use Doctrine\ODM\PHPCR\Mapping\Attributes as PHPCR;
+    use Doctrine\ODM\PHPCR\ReferenceManyCollection;
 
     #[PHPCR\Document]
     class User
@@ -525,7 +526,7 @@ contains a collection of groups::
         #[PHPCR\ReferenceMany]
         private $groups;
 
-        public function getGroups()
+        public function getGroups(): ReferenceManyCollection
         {
             return $this->groups;
         }
diff --git a/docs/en/reference/introduction.rst b/docs/en/reference/introduction.rst
index 4fe8536a4..f527d1ae6 100644
--- a/docs/en/reference/introduction.rst
+++ b/docs/en/reference/introduction.rst
@@ -333,6 +333,12 @@ we can traverse them::
     Children can be of any class. Be careful when looping over children
     to be sure they are of the expected class.
 
+The children collection supports iterating over the children and also direct
+access by index or child name. To get a segment of the collection, you have the
+normal ``slice()`` that works on indexes. Additionally, the
+``ChildrenCollection`` offers the ``sliceByChildName()`` method, where you can
+specify the child name from which to start the slice.
+
 Even if children are not mapped, you can use the document manager to get all
 flushed children of a document::
 
@@ -367,6 +373,7 @@ Lets look at an example of document ``A`` referencing ``B``::
     namespace Demo;
 
     use Doctrine\ODM\PHPCR\Mapping\Attributes as PHPCR;
+    use Doctrine\ODM\PHPCR\ReferrersCollection;
 
     #[PHPCR\Document]
     class A
@@ -381,7 +388,7 @@ Lets look at an example of document ``A`` referencing ``B``::
     class B
     {
         #[PHPCR\Referrers(referringDocument: A::class, referencedBy: 'ref')]
-        private $referrers;
+        private ReferrersCollection $referrers;
     }
 
 We can now create a reference with the following code::
diff --git a/lib/Doctrine/ODM/PHPCR/ChildrenCollection.php b/lib/Doctrine/ODM/PHPCR/ChildrenCollection.php
index 5d1a59263..427f64883 100644
--- a/lib/Doctrine/ODM/PHPCR/ChildrenCollection.php
+++ b/lib/Doctrine/ODM/PHPCR/ChildrenCollection.php
@@ -17,10 +17,7 @@ class ChildrenCollection extends PersistentCollection
 {
     private object $document;
 
-    /**
-     * @var array|string|null
-     */
-    private $filter;
+    private string|array|null $filter;
 
     private int $fetchDepth;
 
@@ -34,14 +31,14 @@ class ChildrenCollection extends PersistentCollection
     /**
      * Creates a new persistent collection.
      *
-     * @param object       $document   The parent document instance
-     * @param string|array $filter     Filter string or array of filter string
-     * @param int          $fetchDepth Optional fetch depth, -1 to not override
-     * @param string|null  $locale     The locale to use during the loading of this collection
+     * @param object            $document   The parent document instance
+     * @param array|string|null $filter     Filter string for child names or array of filter strings
+     * @param int               $fetchDepth Optional fetch depth, -1 to not override
+     * @param string|null       $locale     The locale to use during the loading of this collection
      */
-    public function __construct(DocumentManagerInterface $dm, object $document, $filter = null, int $fetchDepth = -1, string $locale = null)
+    public function __construct(DocumentManagerInterface $dm, object $document, array|string $filter = null, int $fetchDepth = -1, string $locale = null)
     {
-        $this->dm = $dm;
+        parent::__construct($dm);
         $this->document = $document;
         $this->filter = $filter;
         $this->fetchDepth = $fetchDepth;
@@ -159,20 +156,13 @@ public function isEmpty(): bool
         return parent::isEmpty();
     }
 
-    public function slice($offset, $length = null)
+    public function slice(int $offset, $length = null): array
     {
         if (!$this->isInitialized()) {
             $nodeNames = $this->getOriginalNodeNames();
-            if (!is_numeric($offset)) {
-                $offset = array_search($offset, $nodeNames, true);
-                if (false === $offset) {
-                    return new ArrayCollection();
-                }
-            }
-
             $nodeNames = array_slice($nodeNames, $offset, $length);
             $parentPath = $this->getNode($this->fetchDepth)->getPath();
-            array_walk($nodeNames, function (&$nodeName) use ($parentPath) {
+            array_walk($nodeNames, static function (&$nodeName) use ($parentPath) {
                 $nodeName = "$parentPath/$nodeName";
             });
 
@@ -181,12 +171,25 @@ public function slice($offset, $length = null)
             return $this->getChildren($childNodes);
         }
 
-        if (!is_numeric($offset)) {
-            $nodeNames = $this->collection->getKeys();
-            $offset = array_search($offset, $nodeNames, true);
+        return parent::slice($offset, $length);
+    }
+
+    public function sliceByChildName(string $firstChild, $length = null): array
+    {
+        if (!$this->isInitialized()) {
+            $nodeNames = $this->getOriginalNodeNames();
+            $offset = array_search($firstChild, $nodeNames, true);
             if (false === $offset) {
-                return new ArrayCollection();
+                return [];
             }
+
+            return $this->slice($offset, $length);
+        }
+
+        $nodeNames = $this->collection->getKeys();
+        $offset = array_search($firstChild, $nodeNames, true);
+        if (false === $offset) {
+            return [];
         }
 
         return parent::slice($offset, $length);
diff --git a/lib/Doctrine/ODM/PHPCR/Configuration.php b/lib/Doctrine/ODM/PHPCR/Configuration.php
index 4ddd98d01..7d19fc19f 100644
--- a/lib/Doctrine/ODM/PHPCR/Configuration.php
+++ b/lib/Doctrine/ODM/PHPCR/Configuration.php
@@ -25,7 +25,6 @@ class Configuration
         'metadataDriverImpl' => null,
         'metadataCacheImpl' => null,
         'documentClassMapper' => null,
-        'documentNamespaces' => [],
         'proxyNamespace' => 'MyPHPCRProxyNS',
         'autoGenerateProxyClasses' => true,
     ];
@@ -62,38 +61,6 @@ public function getWriteDoctrineMetadata(): bool
         return $this->attributes['writeDoctrineMetadata'];
     }
 
-    /**
-     * Adds a namespace with the specified alias.
-     */
-    public function addDocumentNamespace(string $alias, string $namespace): void
-    {
-        $this->attributes['documentNamespaces'][$alias] = $namespace;
-    }
-
-    /**
-     * Resolves a registered namespace alias to the full namespace.
-     *
-     * @throws PHPCRException
-     */
-    public function getDocumentNamespace(string $documentNamespaceAlias): string
-    {
-        if (!array_key_exists($documentNamespaceAlias, $this->attributes['documentNamespaces'])) {
-            throw PHPCRException::unknownDocumentNamespace($documentNamespaceAlias);
-        }
-
-        return trim($this->attributes['documentNamespaces'][$documentNamespaceAlias], '\\');
-    }
-
-    /**
-     * Set the document alias map.
-     *
-     * @param array<string, string> $documentNamespaces
-     */
-    public function setDocumentNamespaces(array $documentNamespaces): void
-    {
-        $this->attributes['documentNamespaces'] = $documentNamespaces;
-    }
-
     /**
      * Sets the driver implementation that is used to retrieve mapping metadata.
      */
diff --git a/lib/Doctrine/ODM/PHPCR/Decorator/DocumentManagerDecorator.php b/lib/Doctrine/ODM/PHPCR/Decorator/DocumentManagerDecorator.php
index b15306f1a..7064623c9 100644
--- a/lib/Doctrine/ODM/PHPCR/Decorator/DocumentManagerDecorator.php
+++ b/lib/Doctrine/ODM/PHPCR/Decorator/DocumentManagerDecorator.php
@@ -163,7 +163,7 @@ public function reorder(object $document, string $srcName, string $targetName, b
         $this->wrapped->reorder($document, $srcName, $targetName, $before);
     }
 
-    public function getChildren(object $document, $filter = null, int $fetchDepth = -1, string $locale = null): ChildrenCollection
+    public function getChildren(object $document, array|string $filter = null, int $fetchDepth = -1, string $locale = null): ChildrenCollection
     {
         return $this->wrapped->getChildren($document, $filter, $fetchDepth, $locale);
     }
@@ -173,12 +173,12 @@ public function getReferrers(object $document, string $type = null, string $name
         return $this->wrapped->getReferrers($document, $type, $name, $locale, $refClass);
     }
 
-    public function flush($document = null): void
+    public function flush(object|array $document = null): void
     {
         $this->wrapped->flush($document);
     }
 
-    public function getReference(string $documentName, $id)
+    public function getReference(string $documentName, object|string $id): mixed
     {
         return $this->wrapped->getReference($documentName, $id);
     }
diff --git a/lib/Doctrine/ODM/PHPCR/DocumentManager.php b/lib/Doctrine/ODM/PHPCR/DocumentManager.php
index b94af35ee..df639fb4f 100644
--- a/lib/Doctrine/ODM/PHPCR/DocumentManager.php
+++ b/lib/Doctrine/ODM/PHPCR/DocumentManager.php
@@ -478,37 +478,6 @@ public function remove($document): void
         $this->unitOfWork->scheduleRemove($document);
     }
 
-    /**
-     * {@inheritdoc}
-     *
-     * Merge the state of the detached object into the persistence context of
-     * this ObjectManager and returns the managed copy of the object.
-     *
-     * This will copy all fields of $document over the fields of the managed
-     * document and then cascade the merge to relations as configured.
-     *
-     * The object passed to merge will *not* become associated/managed with
-     * this ObjectManager.
-     *
-     * @param object $document the document to merge over a persisted document
-     *                         with the same id
-     *
-     * @return object The managed document where $document has been merged
-     *                into. This is *not* the same instance as the parameter.
-     *
-     * @throws InvalidArgumentException if $document is not an object
-     */
-    public function merge($document): object
-    {
-        if (!is_object($document)) {
-            throw new InvalidArgumentException('Parameter $document needs to be an object, '.gettype($document).' given');
-        }
-
-        $this->errorIfClosed();
-
-        return $this->unitOfWork->merge($document);
-    }
-
     /**
      * {@inheritdoc}
      *
@@ -552,7 +521,7 @@ public function refresh($document): void
         $this->unitOfWork->refresh($document);
     }
 
-    public function getChildren(object $document, $filter = null, int $fetchDepth = -1, string $locale = null): ChildrenCollection
+    public function getChildren(object $document, array|string $filter = null, int $fetchDepth = -1, string $locale = null): ChildrenCollection
     {
         $this->errorIfClosed();
 
@@ -578,7 +547,7 @@ public function getReferrers(object $document, string $type = null, string $name
      * @throws InvalidArgumentException if $document is neither null nor a
      *                                  document or an array of documents
      */
-    public function flush($document = null): void
+    public function flush(object|array $document = null): void
     {
         if (null !== $document && !is_object($document) && !is_array($document)) {
             throw new InvalidArgumentException('Parameter $document needs to be an object, '.gettype($document).' given');
@@ -588,7 +557,7 @@ public function flush($document = null): void
         $this->unitOfWork->commit($document);
     }
 
-    public function getReference(string $documentName, $id)
+    public function getReference(string $documentName, object|string $id): mixed
     {
         return $this->unitOfWork->getOrCreateProxy($id, $documentName);
     }
diff --git a/lib/Doctrine/ODM/PHPCR/DocumentManagerInterface.php b/lib/Doctrine/ODM/PHPCR/DocumentManagerInterface.php
index 80e443a9d..1280d66d5 100644
--- a/lib/Doctrine/ODM/PHPCR/DocumentManagerInterface.php
+++ b/lib/Doctrine/ODM/PHPCR/DocumentManagerInterface.php
@@ -39,18 +39,18 @@
 use PHPCR\Util\QOM\QueryBuilder as PhpcrQueryBuilder;
 
 /**
- * The PHPCR-ODM document manager adds PHPCR specific methods to the object manager..
+ * The PHPCR-ODM document manager adds PHPCR specific methods to the object manager.
  */
 interface DocumentManagerInterface extends ObjectManager
 {
     /**
      * @{@inheritDoc}
      *
-     * Overwritten to tighten return type.
+     * Overwritten to tighten return type. We can't tighten the return type declaration because of Doctrine\Persistence\ObjectManagerDecorator.
      *
      * @return PhpcrClassMetadata
      */
-    public function getClassMetadata($className);
+    public function getClassMetadata(string $className);
 
     /**
      * Add or replace a translation strategy.
@@ -141,7 +141,7 @@ public function findMany(?string $className, array $ids): Collection;
      * @param string      $locale    The language to try to load
      * @param bool        $fallback  Set to true if the language fallback mechanism should be used
      *
-     * @return object the translated document or null if not found
+     * @return object|null the translated document or null if not found
      *
      * @throws MissingTranslationException if $fallback is false and the
      *                                     translation was not found
@@ -301,16 +301,16 @@ public function reorder(object $document, string $srcName, string $targetName, b
      *
      * Note that this method only returns children that have been flushed.
      *
-     * @param object       $document   Document instance which children should be loaded
-     * @param string|array $filter     Optional filter to filter on children names
-     * @param int          $fetchDepth Optional fetch depth
-     * @param string|null  $locale     The locale to use during the loading of this collection
+     * @param object            $document   Document instance which children should be loaded
+     * @param array|string|null $filter     Optional filter to filter on children names
+     * @param int               $fetchDepth Optional fetch depth
+     * @param string|null       $locale     The locale to use during the loading of this collection
      *
      * @return ChildrenCollection collection of child documents
      *
      * @throws InvalidArgumentException if $document is not an object
      */
-    public function getChildren(object $document, $filter = null, int $fetchDepth = -1, string $locale = null): ChildrenCollection;
+    public function getChildren(object $document, array|string $filter = null, int $fetchDepth = -1, string $locale = null): ChildrenCollection;
 
     /**
      * Get the documents that refer a given document using an optional name.
@@ -342,11 +342,9 @@ public function getReferrers(object $document, string $type = null, string $name
      * has its identifier populated. Otherwise a proxy is returned that automatically
      * loads itself on first access.
      *
-     * @param string|object $id
-     *
      * @return mixed|object the document reference
      */
-    public function getReference(string $documentName, $id);
+    public function getReference(string $documentName, object|string $id): mixed;
 
     /**
      * Create a new version of the document that has been previously persisted
@@ -433,7 +431,7 @@ public function getAllLinearVersions(object $document, int $limit = -1): array;
      * @param string $id          Id of the document
      * @param string $versionName The version name as given by getLinearPredecessors
      *
-     * @return object The detached document or null if the document is not found
+     * @return object|null The detached document or null if the document is not found
      *
      * @throws UnsupportedRepositoryOperationException if the implementation does not support versioning
      * @throws InvalidArgumentException                if there is a document with $id but no version with $name
@@ -461,7 +459,7 @@ public function getUnitOfWork(): UnitOfWork;
      * @throws InvalidArgumentException if $document is neither null nor a
      *                                  document or an array of documents
      */
-    public function flush($document = null): void;
+    public function flush(object|array $document = null): void;
 
     /**
      * Closes the DocumentManager. All entities that are currently managed
diff --git a/lib/Doctrine/ODM/PHPCR/ImmutableReferrersCollection.php b/lib/Doctrine/ODM/PHPCR/ImmutableReferrersCollection.php
index fcea9741e..dc55db3ee 100644
--- a/lib/Doctrine/ODM/PHPCR/ImmutableReferrersCollection.php
+++ b/lib/Doctrine/ODM/PHPCR/ImmutableReferrersCollection.php
@@ -17,37 +17,37 @@ public function __construct(DocumentManagerInterface $dm, $document, $type = nul
         parent::__construct($dm, $document, $type, null, $locale);
     }
 
-    public function add($element): bool
+    public function add($element): never
     {
         throw new BadMethodCallException('Can not call '.__METHOD__.' on immutable collection');
     }
 
-    public function clear(): void
+    public function clear(): never
     {
         throw new BadMethodCallException('Can not call '.__METHOD__.' on immutable collection');
     }
 
-    public function offsetSet($offset, $value): void
+    public function offsetSet($offset, $value): never
     {
         throw new BadMethodCallException('Can not call '.__METHOD__.' on immutable collection');
     }
 
-    public function offsetUnset($offset): void
+    public function offsetUnset($offset): never
     {
         throw new BadMethodCallException('Can not call '.__METHOD__.' on immutable collection');
     }
 
-    public function remove($key)
+    public function remove($key): never
     {
         throw new BadMethodCallException('Can not call '.__METHOD__.' on immutable collection');
     }
 
-    public function removeElement($element): bool
+    public function removeElement($element): never
     {
         throw new BadMethodCallException('Can not call '.__METHOD__.' on immutable collection');
     }
 
-    public function set($key, $value): void
+    public function set($key, $value): never
     {
         throw new BadMethodCallException('Can not call '.__METHOD__.' on immutable collection');
     }
diff --git a/lib/Doctrine/ODM/PHPCR/Mapping/ClassMetadataFactory.php b/lib/Doctrine/ODM/PHPCR/Mapping/ClassMetadataFactory.php
index ddddb1bb3..04d0818a9 100644
--- a/lib/Doctrine/ODM/PHPCR/Mapping/ClassMetadataFactory.php
+++ b/lib/Doctrine/ODM/PHPCR/Mapping/ClassMetadataFactory.php
@@ -69,11 +69,6 @@ protected function newClassMetadataInstance($className): ClassMetadata
         return new ClassMetadata($className);
     }
 
-    protected function getFqcnFromAlias($namespaceAlias, $simpleClassName): string
-    {
-        return $this->dm->getConfiguration()->getDocumentNamespace($namespaceAlias).'\\'.$simpleClassName;
-    }
-
     /**
      * Actually loads PHPCR-ODM metadata from the underlying metadata.
      *
diff --git a/lib/Doctrine/ODM/PHPCR/PersistentCollection.php b/lib/Doctrine/ODM/PHPCR/PersistentCollection.php
index 581b7b06f..b5a48287a 100644
--- a/lib/Doctrine/ODM/PHPCR/PersistentCollection.php
+++ b/lib/Doctrine/ODM/PHPCR/PersistentCollection.php
@@ -59,12 +59,12 @@ public function unwrap(): Collection
         return new ArrayCollection();
     }
 
-    public function add($element): bool
+    public function add($element): void
     {
         $this->initialize();
         $this->isDirty = true;
 
-        return $this->collection->add($element);
+        $this->collection->add($element);
     }
 
     public function clear(): void
@@ -95,7 +95,7 @@ public function count(): int
         return $this->collection->count();
     }
 
-    public function current()
+    public function current(): mixed
     {
         $this->initialize();
 
@@ -116,13 +116,27 @@ public function filter(\Closure $p): Collection
         return $this->collection->filter($p);
     }
 
-    public function first()
+    public function first(): mixed
     {
         $this->initialize();
 
         return $this->collection->first();
     }
 
+    public function findFirst(\Closure $p): mixed
+    {
+        $this->initialize();
+
+        return $this->collection->findFirst($p);
+    }
+
+    public function reduce(\Closure $func, $initial = null)
+    {
+        $this->initialize();
+
+        return $this->collection->reduce($func);
+    }
+
     public function forAll(\Closure $p): bool
     {
         $this->initialize();
@@ -130,7 +144,7 @@ public function forAll(\Closure $p): bool
         return $this->collection->forAll($p);
     }
 
-    public function get($key)
+    public function get($key): mixed
     {
         $this->initialize();
 
@@ -158,7 +172,7 @@ public function getValues(): array
         return $this->collection->getValues();
     }
 
-    public function indexOf($element)
+    public function indexOf($element): bool|int|string
     {
         $this->initialize();
 
@@ -172,14 +186,14 @@ public function isEmpty(): bool
         return $this->collection->isEmpty();
     }
 
-    public function key()
+    public function key(): int|null|string
     {
         $this->initialize();
 
         return $this->collection->key();
     }
 
-    public function last()
+    public function last(): mixed
     {
         $this->initialize();
 
@@ -193,7 +207,7 @@ public function map(\Closure $func): Collection
         return $this->collection->map($func);
     }
 
-    public function next()
+    public function next(): mixed
     {
         $this->initialize();
 
@@ -207,8 +221,7 @@ public function offsetExists($offset): bool
         return $this->collection->offsetExists($offset);
     }
 
-    #[\ReturnTypeWillChange] // type mixed is not available for older php versions
-    public function offsetGet($offset)
+    public function offsetGet($offset): mixed
     {
         $this->initialize();
 
@@ -238,7 +251,7 @@ public function partition(\Closure $p): array
         return $this->collection->partition($p);
     }
 
-    public function remove($key)
+    public function remove($key): mixed
     {
         $this->initialize();
         $this->isDirty = true;
@@ -261,7 +274,7 @@ public function set($key, $value): void
         $this->collection->set($key, $value);
     }
 
-    public function slice($offset, $length = null)
+    public function slice(int $offset, $length = null): array
     {
         $this->initialize();
 
@@ -322,10 +335,9 @@ public function setLocale(?string $locale): void
     }
 
     /**
-     * @param array|Collection $collection     The collection to initialize with
-     * @param bool             $forceOverwrite If to force the database to be forced to the state of the collection
+     * @param bool $forceOverwrite Whether to force the database to be updated with $collection
      */
-    protected function initializeFromCollection($collection, bool $forceOverwrite = false): void
+    protected function initializeFromCollection(array|Collection $collection, bool $forceOverwrite = false): void
     {
         $this->collection = is_array($collection) ? new ArrayCollection($collection) : $collection;
         $this->initialized = $forceOverwrite ? self::INITIALIZED_FROM_COLLECTION_FORCE : self::INITIALIZED_FROM_COLLECTION;
diff --git a/tests/Doctrine/Tests/ODM/PHPCR/ConfigurationTest.php b/tests/Doctrine/Tests/ODM/PHPCR/ConfigurationTest.php
deleted file mode 100644
index 36a6ca26d..000000000
--- a/tests/Doctrine/Tests/ODM/PHPCR/ConfigurationTest.php
+++ /dev/null
@@ -1,33 +0,0 @@
-<?php
-
-namespace Doctrine\Tests\ODM\PHPCR;
-
-use Doctrine\ODM\PHPCR\Configuration;
-use Doctrine\ODM\PHPCR\PHPCRException;
-
-/**
- * @group unit
- */
-class ConfigurationTest extends PHPCRTestCase
-{
-    /**
-     * @covers \Doctrine\ODM\PHPCR\Configuration::addDocumentNamespace
-     * @covers \Doctrine\ODM\PHPCR\Configuration::getDocumentNamespace
-     * @covers \Doctrine\ODM\PHPCR\Configuration::setDocumentNamespaces
-     */
-    public function testDocumentNamespace(): void
-    {
-        $config = new Configuration();
-
-        $config->addDocumentNamespace('foo', 'Documents\Bar');
-        $this->assertEquals('Documents\Bar', $config->getDocumentNamespace('foo'));
-
-        $config = new Configuration();
-
-        $config->setDocumentNamespaces(['foo' => 'Documents\Bar']);
-        $this->assertEquals('Documents\Bar', $config->getDocumentNamespace('foo'));
-
-        $this->expectException(PHPCRException::class);
-        $config->getDocumentNamespace('bar');
-    }
-}
diff --git a/tests/Doctrine/Tests/ODM/PHPCR/Functional/FindTypeValidationTest.php b/tests/Doctrine/Tests/ODM/PHPCR/Functional/FindTypeValidationTest.php
index b4f98e69e..af9c965b2 100644
--- a/tests/Doctrine/Tests/ODM/PHPCR/Functional/FindTypeValidationTest.php
+++ b/tests/Doctrine/Tests/ODM/PHPCR/Functional/FindTypeValidationTest.php
@@ -64,10 +64,7 @@ public function testFind(): void
 
     public function testFindWithNamespace(): void
     {
-        $config = $this->dm->getConfiguration();
-        $config->addDocumentNamespace('Foobar', 'Doctrine\Tests\ODM\PHPCR\Functional');
-
-        $user = $this->dm->find('Foobar:TypeUser', 'functional/user');
+        $user = $this->dm->find(TypeUser::class, 'functional/user');
         $this->assertNotNull($user);
     }
 
diff --git a/tests/Doctrine/Tests/ODM/PHPCR/Functional/Hierarchy/ChildrenTest.php b/tests/Doctrine/Tests/ODM/PHPCR/Functional/Hierarchy/ChildrenTest.php
index 263ec2778..cc129e38b 100644
--- a/tests/Doctrine/Tests/ODM/PHPCR/Functional/Hierarchy/ChildrenTest.php
+++ b/tests/Doctrine/Tests/ODM/PHPCR/Functional/Hierarchy/ChildrenTest.php
@@ -106,11 +106,11 @@ public function testSliceChildrenCollection(): void
         $this->createChildren();
 
         $parent = $this->dm->find($this->type, '/functional/parent');
-        $collection = $parent->allChildren->slice('Child B', 2);
+        $collection = $parent->allChildren->sliceByChildName('Child B', 2);
         $this->assertEquals(['Child B', 'Child C'], array_keys($collection));
 
         $parent->allChildren->initialize();
-        $collection = $parent->allChildren->slice('Child B', 2);
+        $collection = $parent->allChildren->sliceByChildName('Child B', 2);
         $this->assertEquals(['Child B', 'Child C'], array_keys($collection));
     }
 
@@ -138,8 +138,8 @@ public function testLazyLoading(): void
         $this->assertCount(4, $parent->allChildren);
         $this->assertFalse($parent->aChildren->contains(new ChildrenTestObj()));
         $this->assertTrue($parent->allChildren->containsKey('Child D'));
-        $this->assertEquals('Child B', key($parent->allChildren->slice('Child B', 2)));
-        $this->assertCount(2, $parent->allChildren->slice('Child B', 2));
+        $this->assertEquals('Child B', key($parent->allChildren->sliceByChildName('Child B', 2)));
+        $this->assertCount(2, $parent->allChildren->sliceByChildName('Child B', 2));
         $this->assertFalse($parent->aChildren->isInitialized());
         $this->assertFalse($parent->allChildren->isInitialized());
 
diff --git a/tests/Doctrine/Tests/ODM/PHPCR/Functional/MergeTest.php b/tests/Doctrine/Tests/ODM/PHPCR/Functional/MergeTest.php
deleted file mode 100644
index 1a6f5726c..000000000
--- a/tests/Doctrine/Tests/ODM/PHPCR/Functional/MergeTest.php
+++ /dev/null
@@ -1,241 +0,0 @@
-<?php
-
-namespace Doctrine\Tests\ODM\PHPCR\Functional;
-
-use Doctrine\ODM\PHPCR\DocumentManager;
-use Doctrine\ODM\PHPCR\Exception\InvalidArgumentException;
-use Doctrine\ODM\PHPCR\ReferenceManyCollection;
-use Doctrine\Tests\Models\CMS\CmsArticle;
-use Doctrine\Tests\Models\CMS\CmsGroup;
-use Doctrine\Tests\Models\CMS\CmsTeamUser;
-use Doctrine\Tests\Models\CMS\CmsUser;
-use Doctrine\Tests\ODM\PHPCR\PHPCRFunctionalTestCase;
-use PHPCR\NodeInterface;
-
-class MergeTest extends PHPCRFunctionalTestCase
-{
-    /**
-     * @var DocumentManager
-     */
-    private $dm;
-
-    /**
-     * @var NodeInterface
-     */
-    private $node;
-
-    public function setUp(): void
-    {
-        $this->dm = $this->createDocumentManager([__DIR__]);
-        $this->node = $this->resetFunctionalNode($this->dm);
-        $this->dm->getPhpcrSession()->save();
-    }
-
-    public function testMergeNewDocument(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $mergedUser = $this->dm->merge($user);
-
-        $this->assertNotSame($mergedUser, $user);
-        $this->assertInstanceOf(CmsUser::class, $mergedUser);
-        $this->assertEquals('beberlei', $mergedUser->username);
-        $this->assertEquals($this->node->getPath().'/'.$mergedUser->username, $mergedUser->id, 'Merged new document should have generated path');
-        $this->assertInstanceOf(ReferenceManyCollection::class, $mergedUser->articles);
-    }
-
-    public function testMergeManagedDocument(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $this->dm->persist($user);
-        $this->dm->flush();
-
-        $mergedUser = $this->dm->merge($user);
-
-        $this->assertSame($mergedUser, $user);
-    }
-
-    public function testMergeKnownDocument(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $this->dm->persist($user);
-        $this->dm->flush();
-        $this->dm->clear();
-
-        $mergedUser = $this->dm->merge($user);
-
-        $this->assertNotSame($mergedUser, $user);
-        $this->assertSame($mergedUser->id, $user->id);
-    }
-
-    public function testMergeRemovedDocument(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $this->dm->persist($user);
-        $this->dm->flush();
-
-        $this->dm->remove($user);
-
-        $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage("Removed document detected during merge at '/functional/beberlei'. Cannot merge with a removed document.");
-        $this->dm->merge($user);
-    }
-
-    public function testMergeWithManagedDocument(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $this->dm->persist($user);
-        $this->dm->flush();
-
-        $mergableUser = new CmsUser();
-        $mergableUser->id = $user->id;
-        $mergableUser->username = 'jgalt';
-        $mergableUser->name = 'John Galt';
-
-        $mergedUser = $this->dm->merge($mergableUser);
-
-        $this->assertSame($mergedUser, $user);
-        $this->assertEquals('jgalt', $mergedUser->username);
-        $this->assertInstanceOf(NodeInterface::class, $mergedUser->node);
-    }
-
-    public function testMergeChangeDocumentClass(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $this->dm->persist($user);
-        $this->dm->flush();
-
-        $otherUser = new CmsUser();
-        $otherUser->username = 'lukas';
-        $otherUser->name = 'Lukas';
-
-        $mergableGroup = new CmsGroup();
-        $mergableGroup->id = $user->id;
-        $mergableGroup->name = 'doctrine';
-
-        $this->expectException(InvalidArgumentException::class);
-        $this->dm->merge($mergableGroup);
-    }
-
-    public function testMergeUnknownAssignedId(): void
-    {
-        $doc = new CmsArticle();
-        $doc->id = '/foo';
-
-        $mergedDoc = $this->dm->merge($doc);
-
-        $this->assertNotSame($mergedDoc, $doc);
-        $this->assertSame($mergedDoc->id, $doc->id);
-    }
-
-    public function testMergeWithChild(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $teamuser = new CmsTeamUser();
-        $teamuser->username = 'jwage';
-        $teamuser->name = 'Jonathan Wage';
-        $teamuser->parent = $user;
-        $user->child = $teamuser;
-
-        $this->dm->persist($user);
-        $this->dm->flush();
-
-        $mergableUser = new CmsUser();
-        $mergableUser->username = 'jgalt';
-        $mergableUser->name = 'John Galt';
-        $mergableUser->id = $user->id;
-
-        $mergedUser = $this->dm->merge($mergableUser);
-
-        $this->assertSame($mergedUser, $user);
-        $this->assertEquals('jgalt', $mergedUser->username);
-        $this->assertEquals('jwage', $mergedUser->child->username);
-
-        $this->dm->flush();
-        $mergedUser->children->count();
-
-        $mergableUser = new CmsUser();
-        $mergableUser->id = $user->id;
-        $mergableUser->username = 'dbu';
-        $mergableUser->name = 'David';
-
-        $mergedUser = $this->dm->merge($mergableUser);
-
-        $this->assertSame($mergedUser, $user);
-        $this->assertEquals('dbu', $mergedUser->username);
-        $this->assertEquals('David', $mergedUser->name);
-        $this->assertEquals('jwage', $mergedUser->child->username);
-
-        $this->dm->flush();
-    }
-
-    public function testMergeWithChildren(): void
-    {
-        $user = new CmsUser();
-        $user->username = 'beberlei';
-        $user->name = 'Benjamin';
-
-        $teamuser = new CmsTeamUser();
-        $teamuser->username = 'jwage';
-        $teamuser->name = 'Jonathan Wage';
-        $teamuser->parent = $user;
-
-        $this->dm->persist($user);
-        $this->dm->persist($teamuser);
-        $this->dm->flush();
-        $this->dm->clear();
-
-        $user = $this->dm->find(null, '/functional/beberlei');
-        $this->assertCount(1, $user->children);
-
-        $mergableUser = new CmsUser();
-        $mergableUser->username = 'jgalt';
-        $mergableUser->name = 'John Galt';
-        $mergableUser->id = $user->id;
-
-        $mergedUser = $this->dm->merge($mergableUser);
-
-        $this->assertSame($mergedUser, $user);
-        $this->assertEquals('jgalt', $mergedUser->username);
-        $this->assertCount(1, $mergedUser->children);
-
-        $this->dm->flush();
-        $mergedUser->children->count();
-
-        $mergableUser = new CmsUser();
-        $mergableUser->id = $user->id;
-        $mergableUser->username = 'dbu';
-
-        $mergedUser = $this->dm->merge($mergableUser);
-
-        $this->assertSame($mergedUser, $user);
-        $this->assertEquals('dbu', $mergedUser->username);
-        $this->assertNull($mergedUser->name);
-        $this->assertCount(1, $mergedUser->children);
-
-        $this->dm->flush();
-
-        $mergedUser = $this->dm->find(null, $mergedUser->id);
-        $this->assertCount(1, $mergedUser->children);
-    }
-}
diff --git a/tests/Doctrine/Tests/ODM/PHPCR/Functional/QueryBuilderTest.php b/tests/Doctrine/Tests/ODM/PHPCR/Functional/QueryBuilderTest.php
index bc25b56b8..a2cbb01fa 100644
--- a/tests/Doctrine/Tests/ODM/PHPCR/Functional/QueryBuilderTest.php
+++ b/tests/Doctrine/Tests/ODM/PHPCR/Functional/QueryBuilderTest.php
@@ -114,12 +114,9 @@ public function testFrom(): void
 
     public function testFromWithAlias(): void
     {
-        $config = $this->dm->getConfiguration();
-        $config->addDocumentNamespace('Foobar', 'Doctrine\Tests\Models\Blog');
-
         $qb = $this->createQb();
-        $qb->from()->document('Foobar:User', 'a');
-        // add where to stop rouge documents that havn't been stored in /functional/ from appearing.
+        $qb->from()->document(BlogUser::class, 'a');
+        // add where to stop rouge documents that haven't been stored in /functional/ from appearing.
         $qb->where()->eq()->field('a.status')->literal('query_builder')->end();
         $res = $qb->getQuery()->execute();
         $this->assertCount(2, $res);
diff --git a/tests/Doctrine/Tests/ODM/PHPCR/Functional/Versioning/VersioningTestAbstract.php b/tests/Doctrine/Tests/ODM/PHPCR/Functional/Versioning/VersioningTestAbstract.php
index d02c31bee..4844065f8 100644
--- a/tests/Doctrine/Tests/ODM/PHPCR/Functional/Versioning/VersioningTestAbstract.php
+++ b/tests/Doctrine/Tests/ODM/PHPCR/Functional/Versioning/VersioningTestAbstract.php
@@ -5,7 +5,6 @@
 use Doctrine\ODM\PHPCR\DocumentManager;
 use Doctrine\ODM\PHPCR\Exception\InvalidArgumentException;
 use Doctrine\ODM\PHPCR\Mapping\Attributes as PHPCR;
-use Doctrine\Tests\Models\Versioning\FullVersionableArticle;
 use Doctrine\Tests\Models\Versioning\FullVersionableArticleWithChildren;
 use Doctrine\Tests\Models\Versioning\NonVersionableArticle;
 use Doctrine\Tests\ODM\PHPCR\PHPCRFunctionalTestCase;
@@ -86,24 +85,6 @@ public function testCheckinOnNonVersionableNode(): void
         $this->dm->checkin($contentNode);
     }
 
-    public function testMergeVersionable(): void
-    {
-        $versionableArticle = new FullVersionableArticle();
-        $versionableArticle->setText('very interesting content');
-        $versionableArticle->author = 'greg0ire';
-        $versionableArticle->topic = 'whatever';
-        $versionableArticle->id = '/functional/whatever';
-        $versionableArticle->versionName = 'v1';
-
-        $this->dm->persist($versionableArticle);
-        $this->dm->flush();
-        $this->dm->clear();
-        $versionableArticle->versionName = 'v2';
-
-        $mergedVersionableArticle = $this->dm->merge($versionableArticle);
-        $this->assertEquals('v2', $mergedVersionableArticle->versionName);
-    }
-
     public function testCheckin(): void
     {
         $user = $this->dm->find($this->typeVersion, '/functional/versionTestObj');