The null mapping map's always to NULL, nothing is stored in the database for this. This is mainly useful as a sub-mapping for other mappings.
Two use cases would be to map a deprecated option in a choice-mapping or have null-values inside an array-mapping for specific keys.
<?xml version="1.0" encoding="utf-8"?>
<doctrine-mapping
xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
xmlns:rdm="https://github.com/addiks/symfony_rdm/raw/master/Resources/mapping-schema.v1.xsd"
>
<entity name="Foo\Bar\SomeEntity" table="some_entity">
…
<rdm:array field="data">
<rdm:null field="someEmptyThing" />
…
</rdm:array>
…
<rdm:choice field="thingType" column="thing_type">
<rdm:option name="foo"><rdm:service id="foo_service" /></rdm:option>
<rdm:option name="bar"><rdm:service id="bar_service" /></rdm:option>
<rdm:option name="baz"><rdm:null /></rdm:option>
</rdm:choice>
…
</entity>
</doctrine-mapping>