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backend/vendor/doctrine/collections/docs/en/serialization.rst

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+Serialization
+=============
+
+Using (un-)serialize() on a collection is not a supported use-case
+and may break when changes on the collection's internals happen in the future.
+If a collection needs to be serialized, use ``toArray()`` and reconstruct
+the collection manually.
+
+.. code-block:: php
+
+    $collection = new ArrayCollection([1, 2, 3]);
+    $serialized = serialize($collection->toArray());
+
+A reconstruction is also necessary when the collection contains objects with
+infinite recursion of dependencies like in this ``json_serialize()`` example:
+
+.. code-block:: php
+
+    $foo = new Foo();
+    $bar = new Bar();
+
+    $foo->setBar($bar);
+    $bar->setFoo($foo);
+
+    $collection = new ArrayCollection([$foo]);
+    $json       = json_serialize($collection->toArray()); // recursion detected
+
+Serializer libraries can be used to create the serialization-output to prevent
+errors.