Doctrine2 cascade option SET_NULL is incorrect.
It will result in an Invalid foreign key action: SET_NULL
error. I'm using version 3.2.6.1309.
Updating the Doctrine2.skipper.cfg.xml
solves it!
<!-- http://www.doctrine-project.org/projects/orm/2.0/docs/reference/basic-mapping/en#doctrine-mapping-types -->
<!-- SET_NULL is Invalid foreign key action, so we override it -->
<attribute-types>
<struct name="ManyToManyEntity" section="orm"
help-url="http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/xml-mapping.html#defining-many-to-many-associations">
<attribute name="on-delete" type="enum" enum-values="CASCADE|RESTRICT|SET NULL"
help-text="Cascade Action (Database-level)"
help-url="http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/xml-mapping.html#join-column-element"/>
<attribute name="on-update" type="enum" enum-values="CASCADE|RESTRICT|SET NULL"
help-text="Cascade Action (Database-level). Warning: Removed from version 2.2 (DDC-1628)."
help-url=""/>
</struct>
</attribute-types>
</orm-configuration>