[BAEL-5373] A Guide to @DBRef in MongoDB (#11909)

* Add implementation for BAEL-5373.

* Fix test class name.

* Change collection names to be aligned with article.

persistence-modules/spring-boot-persistence-mongodb/src/main/java/com/baeldung/mongodb/dbref/DbRefApplication.java

@@ -0,0 +1,13 @@
+package com.baeldung.mongodb.dbref;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class DbRefApplication {
+
+    public static void main(String... args) {
+        SpringApplication.run(DbRefApplication.class, args);
+    }
+
+}

persistence-modules/spring-boot-persistence-mongodb/src/main/java/com/baeldung/mongodb/dbref/DbRefTester.java

@@ -0,0 +1,26 @@
+package com.baeldung.mongodb.dbref;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.ApplicationArguments;
+import org.springframework.boot.ApplicationRunner;
+import org.springframework.stereotype.Component;
+
+import com.baeldung.mongodb.dbref.repository.PersonRepository;
+
+@Component
+public class DbRefTester implements ApplicationRunner {
+
+    private static final Logger logger = LoggerFactory.getLogger(DbRefTester.class);
+    
+    @Autowired
+    private PersonRepository personRepository;
+
+    @Override
+    public void run(ApplicationArguments args) throws Exception {
+        logger.info("{}", personRepository.findAll());
+
+    }
+
+}

persistence-modules/spring-boot-persistence-mongodb/src/main/java/com/baeldung/mongodb/dbref/model/Person.java

@@ -0,0 +1,49 @@
+package com.baeldung.mongodb.dbref.model;
+
+import java.util.List;
+
+import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.mapping.DBRef;
+import org.springframework.data.mongodb.core.mapping.Document;
+
+@Document(collection = "Person")
+public class Person {
+
+    @Id
+    private String id;
+
+    private String name;
+
+    @DBRef
+    private List<Pet> pets;
+
+    public String getId() {
+        return id;
+    }
+
+    public void setId(String id) {
+        this.id = id;
+    }
+
+    public List<Pet> getPets() {
+        return pets;
+    }
+
+    public void setPets(List<Pet> pets) {
+        this.pets = pets;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public String toString() {
+        return "Person [id=" + id + ", name=" + name + ", pets=" + pets + "]";
+    }
+
+}

persistence-modules/spring-boot-persistence-mongodb/src/main/java/com/baeldung/mongodb/dbref/model/Pet.java

@@ -0,0 +1,29 @@
+package com.baeldung.mongodb.dbref.model;
+
+public class Pet {
+
+    private String id;
+    private String name;
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public String getId() {
+        return id;
+    }
+
+    public void setId(String id) {
+        this.id = id;
+    }
+
+    @Override
+    public String toString() {
+        return "Pet [id=" + id + ", name=" + name + "]";
+    }
+
+}

persistence-modules/spring-boot-persistence-mongodb/src/main/java/com/baeldung/mongodb/dbref/repository/PersonRepository.java

@@ -0,0 +1,9 @@
+package com.baeldung.mongodb.dbref.repository;
+
+import org.springframework.data.mongodb.repository.MongoRepository;
+
+import com.baeldung.mongodb.dbref.model.Person;
+
+public interface PersonRepository extends MongoRepository<Person, String> {
+
+}

persistence-modules/spring-boot-persistence-mongodb/src/test/java/com/baeldung/mongodb/dbref/DbRefIntegrationTest.java

@@ -0,0 +1,74 @@
+package com.baeldung.mongodb.dbref;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import com.baeldung.mongodb.dbref.model.Person;
+import com.baeldung.mongodb.dbref.model.Pet;
+import com.baeldung.mongodb.dbref.repository.PersonRepository;
+import com.mongodb.BasicDBObjectBuilder;
+import com.mongodb.DBObject;
+import com.mongodb.DBRef;
+
+@RunWith(SpringRunner.class)
+@SpringBootTest
+@DirtiesContext
+public class DbRefIntegrationTest {
+
+    @Autowired
+    PersonRepository personRepository;
+
+    @Autowired
+    private MongoTemplate mongoTemplate;
+
+    @Test
+    public void givenPetsAndPersonInDatabase_whenListPersons_thenReferenceIsFetched() {
+        // given
+        DBObject catInDatabase = BasicDBObjectBuilder.start()
+            .add("name", "Loki")
+            .get();
+
+        DBObject dogInDatabase = BasicDBObjectBuilder.start()
+            .add("name", "Max")
+            .get();
+
+        mongoTemplate.save(catInDatabase, "Cat");
+        mongoTemplate.save(dogInDatabase, "Dog");
+
+        List<DBRef> petsReference = new ArrayList<DBRef>();
+        petsReference.add(new DBRef("Cat", catInDatabase.get("_id")));
+        petsReference.add(new DBRef("Dog", dogInDatabase.get("_id")));
+
+        DBObject personInDatabase = BasicDBObjectBuilder.start()
+            .add("name", "Bob")
+            .add("pets", petsReference)
+            .get();
+
+        mongoTemplate.save(personInDatabase, "Person");
+
+        // when
+        List<Person> persons = personRepository.findAll();
+
+        // then
+        assertThat(persons).hasSize(1);
+        Person person = persons.get(0);
+        assertEquals("Bob", person.getName());
+
+        List<Pet> pets = person.getPets();
+        assertThat(pets).hasSize(2);
+        assertThat(pets).anyMatch(pet -> "Loki".equals(pet.getName()));
+        assertThat(pets).anyMatch(pet -> "Max".equals(pet.getName()));
+    }
+
+}