Can not edit Doctrine ArrayCollection - doctrine-orm

Question

I have a problem with editing an Doctrine2 ArrayCollection of a many-to-many assosciation.
Persisting an new entity is no problem and works fine. But if I would like to persist an entity with new added CollectionItems I got an
Doctrine\ORM\ORMInvalidArgumentException
I use the xml-mapping, the mapping files are the following:
receipt:
<?xml version="1.0" encoding="utf-8"?>
<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">
<entity name="Application\Entity\Receipt" table="receipt" repository-class="Application\Repositories\ReceiptRepository">
<indexes>
<index name="FK_receipt_profession" columns="professsion"/>
<index name="FK_receipt_items_needed" columns="items"/>
</indexes>
<id name="rId" type="integer" column="r_id">
<generator strategy="IDENTITY"/>
</id>
<field name="name" type="string" column="name" length="100" nullable="false"/>
<many-to-one field="profession" target-entity="Application\Entity\Profession">
<join-columns>
<join-column name="profession" referenced-column-name="p_id"/>
</join-columns>
</many-to-one>
<many-to-many field="items" target-entity="Application\Entity\ItemsNeeded" inversed-by="receipt">
<!--<cascade><cascade-all/></cascade>-->
<join-table name="receipt_x_items_needed">
<join-columns>
<join-column name="receipt" referenced-column-name="r_id"/>
</join-columns>
<inverse-join-columns>
<join-column name="itemNeeded" referenced-column-name="in_id"/>
</inverse-join-columns>
</join-table>
</many-to-many>
</entity>
</doctrine-mapping>
ItemsNeeded:
<?xml version="1.0" encoding="utf-8"?>
<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">
<entity name="Application\Entity\ItemsNeeded" table="items_needed" repository-class="Application\Repositories\ItemsneededRepository">
<indexes>
<index name="FK_items_needed_receipt" columns="receipt_id"/>
<index name="item_id" columns="item_id"/>
</indexes>
<id name="inId" type="integer" column="in_id">
<generator strategy="IDENTITY"/>
</id>
<field name="count" type="integer" column="count" nullable="false"/>
<many-to-one field="item" target-entity="Application\Entity\Items">
<join-columns>
<join-column name="item_id" referenced-column-name="i_id"/>
</join-columns>
</many-to-one>
<many-to-many field="receipt" target-entity="Application\Entity\Receipt" mapped-by="items"/>
</entity>
</doctrine-mapping>
and items:
<?xml version="1.0" encoding="utf-8"?>
<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">
<entity name="Application\Entity\Items" table="items" repository-class="Importer\Repository\ItemRepository">
<unique-constraints>
<unique-constraint name="buffed_id" columns="buffed_id"/>
</unique-constraints>
<id name="iId" type="integer" column="i_id">
<generator strategy="IDENTITY"/>
</id>
<field name="name" type="string" column="name" length="100" nullable="false"/>
<field name="buffedId" type="integer" column="buffed_id" nullable="false"/>
</entity>
</doctrine-mapping>
The receipt table has an many-to-many relation through the crosstable "receipt_x_items_needed".
If I call $receipt->addItem($item); and $item is an already existing item from the itemtable I got the following error:
A new entity was found through the relationship 'Application\Entity\ItemsNeeded#item' that was not configured to cascade persist operations for entity: Application\Entity\Items#0000000041f6e49000000000651183aa. To solve this issue: Either explicitly call EntityManager#persist() on this unknown entity or configure cascade persist this association in the mapping for example #ManyToOne(..,cascade={"persist"}). If you cannot find out which entity causes the problem implement 'Application\Entity\Items#__toString()' to get a clue.
But this is no new entity. All data are correctly set and as I wrote, if I add e new receipt all works fine.