Spring data MongoDb: MappingMongoConverter remove _class

So here's the story: we add the type by default as some kind of hint what class to instantiate actually. As you have to pipe in a type to read the document into via MongoTemplate anyway there are two possible options:

1. You hand in a type the actual stored type can be assigned to. In that case we consider the stored type, use that for object creation. Classical example here is doing polymorphic queries. Suppose you have an abstract class Contact and your Person. You could then query for Contacts and we essentially have to determine a type to instantiate.
2. If you - on the other hand - pass in a completely different type we'd simply marshal into that given type, not into the one stored in the document actually. That would cover your question what happens if you move the type.

You might be interested in watching this ticket which covers some kind of pluggable type mapping strategy to turn the type information into an actual type. This can serve simply space saving purposes as you might want to reduce a long qualified class name to a hash of a few letters. It would also allow more complex migration scenarios where you might find completely arbitrary type keys produced by another datastore client and bind those to Java types.

Here's my annotation, and it works.

@Configurationpublic class AppMongoConfig {    public @Bean    MongoDbFactory mongoDbFactory() throws Exception {        return new SimpleMongoDbFactory(new Mongo(), "databasename");    }    public @Bean    MongoTemplate mongoTemplate() throws Exception {        //remove _class        MappingMongoConverter converter = new MappingMongoConverter(mongoDbFactory(), new MongoMappingContext());        converter.setTypeMapper(new DefaultMongoTypeMapper(null));        MongoTemplate mongoTemplate = new MongoTemplate(mongoDbFactory(), converter);        return mongoTemplate;    }}

If you want to disable _class attribute by default, but preserve polymorfism for specified classes, you can explictly define the type of _class (optional) field by configuing:

@Beanpublic MongoTemplate mongoTemplate() throws Exception {    Map<Class<?>, String> typeMapperMap = new HashMap<>();    typeMapperMap.put(com.acme.domain.SomeDocument.class, "role");    TypeInformationMapper typeMapper1 = new ConfigurableTypeInformationMapper(typeMapperMap);    MongoTypeMapper typeMapper = new DefaultMongoTypeMapper(DefaultMongoTypeMapper.DEFAULT_TYPE_KEY, Arrays.asList(typeMapper1));    MappingMongoConverter converter = new MappingMongoConverter(mongoDbFactory(), new MongoMappingContext());    converter.setTypeMapper(typeMapper);    MongoTemplate mongoTemplate = new MongoTemplate(mongoDbFactory(), converter);    return mongoTemplate;}

This will preserve _class field (or whatever you want to name in construtor) for only specified entities.

You can also write own TypeInformationMapper for example based on annotations. If you annotate your document by @DocumentType("aliasName") you will keep polymorphism by keeping alias of class.

I have explained briefly it on my blog, but here is some piece of quick code:https://gist.github.com/athlan/6497c74cc515131e1336