Spring Data with MongoDB support empowers many of applications which uses NoSQL approach to store data. However, it still tries to map abstract raw documents into strongly typed Java Objects.

NoSQL documents brings no information about it’s structure, it keeps simple data like values, arrays and nested documents. In Java code it is possible to make an aggregate (instance of Object in Object) which corresponds nested documents in NoSQL structure. This can save some extra joins or additional queries while One-To-Many and One-To-One relations known from relational data model.

Serializing objects is possible while the object structure is known. While you save the object to database it is simply serialized into JSON string.

There is no problem while you fetch the JSON objects from database and assign to the known Java structure as well. Sometimes in Objective programming approach objects are extended by other classes with extended structure. For example there is object A and object B (extending A without any extra fields). In this point while you query for all objects of type A (type A and B should be returned, because B is also A) there is no way to determine object’s type while deserialization. This is known as polymorphism in Object oriented programming.

To solve that issue Spring Data Mongo Template adds an extra _class field where the canonical name of class is stored.

In many cases we do not use polymorphism in out data model and adding extra _class field is disk space loss.

Avoid storing _class field

To avoid storing _class field, simply innject DefaultMongoTypeMapper with extra class field name set to null in into MongoTemplate:

@Configuration
@EnableMongoRepositories(basePackages="example")
public class AuthDataSourceConfiguration {

	// ...
    
	@Bean
	public MongoClient mongoDbClient() throws Exception {
		return new MongoClient(new ServerAddress("127.0.0.1"));
	}

	@Bean
	public MongoDbFactory mongoDbFactory() throws Exception {
		return new SimpleMongoDbFactory(mongoDbClient(), "dbname");
	}

	@Bean
	public MongoTemplate mongoTemplate() throws Exception {
		MongoTypeMapper typeMapper = new DefaultMongoTypeMapper(null);
        MappingMongoConverter converter = new MappingMongoConverter(mongoDbFactory(), new MongoMappingContext());
        converter.setTypeMapper(typeMapper);
        
		MongoTemplate mongoTemplate = new MongoTemplate(mongoDbFactory(), converter);
		return mongoTemplate;
	}
}

This solution will completely ignore the extra type value.

Caution! You cannot deserialize objects which extends another objects anymore!

Customize _class field value depending on Object type

Another approach is explictly define the value of _class field for specified types. Some types should have an information about type, some not.

We will try achieve:

1. Still keep polymorphism in some cases
2. We will explictly define whether classes should have an info about Java type.
3. The Java type should be configurable and shorter than fully qualified class name and indepedent from source code.
4. The _class field name should be also configured.

Ad1. Ad2. To specify how types are mapped to aliases and vice-versa, you have to implement TypeInformationMapper interface or use one of existing, such us ConfigurableTypeInformationMapper which expects Map<ClassTypeInformation<?>, Object>. This is easy and convenient way to map class to some key (not neccesery a fully qualified class name). If some mapping does not exists, it’s alias is null and will not be inserted into document.

Ad3. The field name is provided by MongoTypeMapper (in our case DefaultMongoTypeMapper):

	// ...
	
	@Bean
	public MongoTemplate mongoTemplate() throws Exception {
		TypeInformationMapper typeMapper1 = ...;
		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;
	}

We assumed default value DefaultMongoTypeMapper.DEFAULT_TYPE_KEY.

We will implement the mechanism based on annotations which scans packages and expects @DocumentType("aliasValue").

First, define our annotation:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface DocumentType {
	
	public String value() default "";

}

And second, create custom TypeInformationMapper based on packages scanning and lookup for annotations:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider;
import org.springframework.core.type.filter.AnnotationTypeFilter;
import org.springframework.data.convert.TypeInformationMapper;
import org.springframework.data.util.ClassTypeInformation;
import org.springframework.data.util.TypeInformation;

/**
 * @author Piotr `Athlan` Pelczar
 */
public class AnnotationTypeInformationMapper implements TypeInformationMapper {

	private final Map, String> typeToAliasMap;
	private final Map> aliasToTypeMap;
	
	private AnnotationTypeInformationMapper(List basePackagesToScan) {
		typeToAliasMap = new HashMap<>();
		aliasToTypeMap = new HashMap<>();
		
		populateTypeMap(basePackagesToScan);
	}
	
	private void populateTypeMap(List basePackagesToScan) {
		ClassPathScanningCandidateComponentProvider scanner = new ClassPathScanningCandidateComponentProvider(false);
		
		scanner.addIncludeFilter(new AnnotationTypeFilter(DocumentType.class));
		
		for (String basePackage : basePackagesToScan) {
			for (BeanDefinition bd : scanner.findCandidateComponents(basePackage)) {
				try {
					Class< ?> clazz = Class.forName(bd.getBeanClassName());
					DocumentType doumentTypeAnnotation = clazz.getAnnotation(DocumentType.class);
					
					ClassTypeInformation< ?> type = ClassTypeInformation.from(clazz);
					String alias = doumentTypeAnnotation.value();
					
					typeToAliasMap.put(type, alias);
					aliasToTypeMap.put(alias, type);
					
				} catch (ClassNotFoundException e) {
					throw new IllegalStateException(String.format("Class [%s] could not be loaded.", bd.getBeanClassName()), e);
				}
			}
		}
	}

	/* 
	 * (non-Javadoc)
	 * @see org.springframework.data.convert.TypeInformationMapper#createAliasFor(org.springframework.data.util.TypeInformation)
	 */
	public Object createAliasFor(TypeInformation< ?> type) {
		ClassTypeInformation< ?> typeClass = (ClassTypeInformation< ?>) type;
		
		if(typeToAliasMap.containsKey(typeClass)) {
			return typeToAliasMap.get(typeClass);
		}
		
		return null;
	}

	/*
	 * (non-Javadoc)
	 * @see org.springframework.data.convert.TypeInformationMapper#resolveTypeFrom(java.lang.Object)
	 */
	public ClassTypeInformation< ?> resolveTypeFrom(Object alias) {

		if(aliasToTypeMap.containsKey(alias)) {
			return aliasToTypeMap.get(alias);
		}
		
		return null;
	}

	public static class Builder {
		List basePackagesToScan;
		
		public Builder() {
			basePackagesToScan = new ArrayList<>();
		}
		
		public Builder withBasePackage(String basePackage) {
			basePackagesToScan.add(basePackage);
			return this;
		}
		
		public Builder withBasePackages(String[] basePackages) {
			basePackagesToScan.addAll(Arrays.asList(basePackages));
			return this;
		}
		
		public Builder withBasePackages(Collection< ? extends String> basePackages) {
			basePackagesToScan.addAll(basePackages);
			return this;
		}
		
		public AnnotationTypeInformationMapper build() {
			AnnotationTypeInformationMapper builded = new AnnotationTypeInformationMapper(basePackagesToScan);
			
			return builded;
		}
	}
}

The usage is simple:

	@Bean
	public MongoTemplate mongoTemplate() throws Exception {
		String[] basePackages = new String[] {"selly"};
		TypeInformationMapper typeMapper1 = new AnnotationTypeInformationMapper.Builder().withBasePackages(basePackages).build();
		
		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;
	}

Complete Gist:
https://gist.github.com/athlan/6497c74cc515131e1336

Hope it helped 🙂