Projections using MongoDB Query in Spring Boot

Projections using MongoDB Query in Spring Boot | By default, the custom query fetches all variables/key data from the collection. But using Projections we can fetch only selected variables/keys from the collection. @Query(value="{ … condition…}", fields =" {…variable : 1.. }"

Here field section is used to select variables for display:-

• One(:1) means display variable.
• Zero(:0) means do not display variables.

Default values for display:-

• Primary Key Variable => default is 1
• Other variables => default is 0

Create a spring starter project with dependencies:- Lombok, Spring Data MongoDB.

In application.properties:-

spring.data.mongodb.host=localhost
spring.data.mongodb.port=27017
# if given database not exist then auto-created
spring.data.mongodb.database=kp

@Data
@NoArgsConstructor
@AllArgsConstructor
@Document
public class Book {
    @Id
    private Integer bookId;
    private String bookName;
    private String bookAuthor;
    private Double bookCost;
    private String bookType;
}

public interface BookRepository extends MongoRepository<Book, String> { }

@Component
@Order(1)
public class MyDataRunner implements CommandLineRunner {

    @Autowired
    private BookRepository bookRepository;

    @Override
    public void run(String... args) throws Exception {
        bookRepository.deleteAll();

        bookRepository.save(new Book(2505, "Spring Boot", "A", 500.0, "BackEnd"));
        bookRepository.save(new Book(2506, "Microservice", "B", 600.0, "BackEnd"));
        bookRepository.save(new Book(2507, "Angular", "C", 700.0, "FrontEnd"));
    }

}

Spring Boot MongoDB Projections Example

// SQL: select bookName, bookAuthor from book where bid>?
@Query(value = "{bookId : {$gt : ?0}}", fields = "{bookName: 1, bookAuthor:1}")
List<Book> getAllBooksByIdProjections(Integer id);

repo.getAllBooksByIdProjections(100).forEach(System.out::println);

Output:-

Book(bookId=2505, bookName=Spring Boot, bookAuthor=A, bookCost=null, bookType=null)
Book(bookId=2506, bookName=Microservice, bookAuthor=B, bookCost=null, bookType=null)
Book(bookId=2507, bookName=Angular, bookAuthor=C, bookCost=null, bookType=null)

We have used List as the return type therefore the unavailable fields like bookCost, and bookType are null. We can also use JSON string for the return type.

@Query(value = "{bookId : {$gt : ?0}}", fields = "{bookName: 1, bookAuthor:1}")
List<String> getAllBooksByIdProjections(Integer id);

Output:-

{“_id”: 2505, “bookName”: “Spring Boot”, “bookAuthor”: “A”}
{“_id”: 2506, “bookName”: “Microservice”, “bookAuthor”: “B”}
{“_id”: 2507, “bookName”: “Angular”, “bookAuthor”: “C”}

Output (Return Type of method) Custom Query for Project is only two Types:-

• String
• ClassType

Example: List, String, List, and Book are allowed.
Here, Object[ ], Double, etc are not allowed.

package com.knowprogram.demo.repo;

import java.util.List;

import org.springframework.data.mongodb.repository.MongoRepository;
import org.springframework.data.mongodb.repository.Query;

import com.knowprogram.demo.model.Book;

public interface BookRepository extends MongoRepository<Book, String> {
    // SQL: select * from book where bid>?
    @Query("{bookId : {$gt : ?0}}")
    List<Book> getAllBooksById(Integer id);

    // SQL: select bookName, bookAuthor from book where bid>?
    @Query(value = "{bookId : {$gt : ?0}}", fields = "{bookName: 1, bookAuthor:1}")
    List<String> getAllBooksByIdProjections(Integer id);

    // SQL: select bookAuthor fom book where bookType=?
    @Query(value = "{bookType: ?0}", fields = "{bookAuthor:1, _id:0}")
    List<String> getBooksDataA(String bookType);

    // SQL: select bookName, bookAuthor from book where bookName like ?
    @Query(value = "{bookName: {$regex: ?0}}", fields = "{bookName: 1, bookAuthor:1}")
    List<String> getBooksByBookName(String input);

    // SQL: select bookName, bookAuthor from book where bookId In (...)
    @Query(value = "{bookId: {$in: ?0}}", fields = "{bookName: 1, bookAuthor:1}")
    List<String> getBooksByBookIds(List<Integer> ids);

    // SQL: select bookName, bookAuthor from book where bookId In (...)
    @Query(value = "{bookId: {$gt: ?0, $lt: ?1}}", fields = "{bookName: 1, bookAuthor:1}")
    List<String> getBooksByBookIdBetween(Integer gt, Integer lt);

    // SQL: SELECT count(*) FROM BOOK WHERE bookId >?
    @Query(value = "{bookId: {$gt: ?0}}", count = true)
    Integer getBooksCount(Integer id);

    // SQL: SELECT * FROM BOOK WHERE bookId >? order by bookType
    // @Query(value = "{bookId: {$gt:?0}}", sort = "{bookType: 1}") // ascending
    // order
    @Query(value = "{bookId: {$gt:?0}}", sort = "{bookType: -1}") // descending order
    List<Book> getBooksInOrder(Integer id);

    @Query(value = "{bookId: {$gt:?0}}", delete = true)
    Long deleteBooksById(Integer id);
}

@Component
@Order(2)
public class DataTestRunner implements CommandLineRunner {

    @Autowired
    private BookRepository repo;

    @Override
    public void run(String... args) throws Exception {
        repo.getAllBooksByIdProjections(100)
            .forEach(System.out::println);

        repo.getBooksDataA("BackEnd")
            .forEach(System.out::println);

        // repo.getBooksByBookName("^S") // starts with ^<input>
        // repo.getBooksByBookName("e$") // ends with <input>$
        repo.getBooksByBookName("o") // containing
            .forEach(System.out::println);

        repo.getBooksByBookIds(Arrays.asList(2505, 2506))
            .forEach(System.out::println);
        
        repo.getBooksByBookIdBetween(2500, 2900)
            .forEach(System.out::println);

        System.out.println(repo.getBooksCount(2500)); // 3
        System.out.println(repo.getBooksInOrder(2500));
        System.out.println(repo.deleteBooksById(2500));
    }

}

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