What is this field?

When Spring Boot integrates with MongoDB via spring-data-mongodb, saving data automatically inserts field _class. Without extra configuration, object saving includes class package information as below.

> db.article.find()
{
  "_class": "com.madplay.model.Article",
  "_id": "ObjectId("153e830031fv4af2bd16e"),
  "title": "Hello, MadPlay!"
}

Then what is _class for? MongoDB collections can store various instance types as documents. To reconstruct fetched documents into complete objects, type information must be read correctly. So Spring stores type metadata during save, and _class acts as that hint.

Can we avoid storing _class?

Yes. With reactive MongoDB dependency spring-boot-starter-data-mongodb-reactive, configure as below. Other parts vary by project, but the key is DefaultMongoTypeMapper. With this setting, _class is not stored.

@Configuration
@EnableReactiveMongoRepositories(basePackages = "com.madplay.taengtest",
        reactiveMongoTemplateRef = "simpleReactiveMongoTemplate")
public class MongoConfiguration {

	private final MongoMappingContext mongoMappingContext;

	public MongoConfiguration(MongoMappingContext mongoMappingContext) {
		this.mongoMappingContext = mongoMappingContext;
	}

    @Bean
    public ReactiveMongoDatabaseFactory reactiveMongoDatabaseFactory(MongoClient mongoClient) {
        return new SimpleReactiveMongoDatabaseFactory(mongoClient, "taeng");
    }

    @Bean
    public MappingMongoConverter reactiveMappingMongoConverter() {
        MappingMongoConverter converter = new MappingMongoConverter(ReactiveMongoTemplate.NO_OP_REF_RESOLVER,
            mongoMappingContext);

		// this line is the key: removes '_class' field
        converter.setTypeMapper(new DefaultMongoTypeMapper(null));
        return converter;
    }

    @Bean
    public ReactiveMongoTemplate simpleReactiveMongoTemplate(ReactiveMongoDatabaseFactory reactiveMongoDatabaseFactory, 
        MappingMongoConverter reactiveMappingMongoConverter) {
        return new ReactiveMongoTemplate(reactiveMongoDatabaseFactory, reactiveMappingMongoConverter);
    }
}

Can _class value be changed instead?

By default, _class stores the full entity class path including package. You can assign a different value instead of this default. Use @TypeAlias as below.

// _class field is stored as `taeng`
@TypeAlias("taeng")
class Person {
	// omitted
}