Spring Data MongoDB add _class column

Question

I am using MappingMongoConverter for converting an object to DBObject. I need the _class field to be added to the converted object.

Bean code:

<bean id="mappingMongoConverter" class="org.springframework.data.mongodb.core.convert.MappingMongoConverter">
    <constructor-arg name="mongoDbFactory" ref="mongoDbFactory" />
    <constructor-arg name="mappingContext" ref="mappingContext" />
</bean>

Java code:

// Here springJavaToMongoConverter is autowired to mappingMongoConverter bean
DBObject convertedObject = (DBObject) springJavaToMongoConverter.convertToMongoType(domainTransactionDTO);

The converted DBObject created does not have _class field.

Question: How do i make sure that the _class field is added automatically on conversion?

Thanks in advance.