How to retrieve an embedded list of object of Entity? - mongodb

Question

I have a simple problem storing and retrieving an embedded collection of entity to mongo. I have checked theses question :
how to serialize class? and Mongodb saves list of object
what I understand is to save a list objects the class of that objects must extends ReflactionDBObject. This worked for saving the object, by retrieving it with the embedded collection does not work.
here a simple test show that retrieving embedded entities does not work !
#Test
public void whatWasStoredAsEmbeddedCollectionIsRetrieved2() {
BasicDBObject country = new BasicDBObject();
country.put("name", "Bulgaria");
List<City> cities = Lists.newArrayList(new City("Tarnovo"));
country.put("listOfCities", cities);
DBCollection collection = db().get().getCollection("test_Collection");
collection.save(country);
DBCursor object = collection.find(new BasicDBObject().append("name", "Bulgaria"));
DBObject returnedCity = object.next();
DBObject embeddedCities = (DBObject) returnedCity.get("listOfCities");
System.out.println(embeddedCities);
}
Here is the City Class
class City extends ReflectionDBObject {
String name;
City() {
}
City(String name) {
this.name = name;
}
public String getName() {
return name;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof City)) return false;
City city = (City) o;
if (name != null ? !name.equals(city.name) : city.name != null) return false;
return true;
}
#Override
public int hashCode() {
return name != null ? name.hashCode() : 0;
}
#Override
public String toString() {
return "City{" +
"name='" + name + '\'' +
'}';
}
}
The out put of the System.out.println statement is [ { "_id" : null }]
Now how can get back the embedded object and the embedded list in it ?

Answer 1

If you do not have a requirement to define your own class City, you can define subdocuments using the BasicDBObjects. I only added the 'name' field to the citySubDoc1 and citySubDoc2, but of course, you can add more fields to these subdocuments.
// Define subdocuments
BasicDBObject citySubDoc1 = new BasicDBObject();
citySubDoc1.put("name", "Tarnovo");
BasicDBObject citySubDoc2 = new BasicDBObject();
citySubDoc2.put("name", "Sofia");
// add to list
List<DBObject> cities = new ArrayList <DBObject>();
cities.add(citySubDoc1);
cities.add(citySubDoc2);
country.put("listOfCities", cities);
collection.save(country);
// Specify query condition
BasicDBObject criteriaQuery = new BasicDBObject();
criteriaQuery.put("name", "Bulgaria");
// Perform the read
DBCursor cursor = collection.find(criteriaQuery);
// Loop through the results
try {
while (cursor.hasNext()) {
List myReturnedListOfCities = (List) cursor.next().get("listOfCities");
System.out.println(myReturnedListOfCities);
}
} finally {
cursor.close();
}