[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/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()));
+    }
+
+}