MongoDB Java: Finding objects in Mongo using QueryBuilder $in operator returns nothing

java, mongo-java, mongodb

Solution

You are now doing the equivalent of :

db.col.find({$in:[{Id:id1}, {Id:id2}, ..., {Id:idN}]})

Which is not a valid query since you're not specifying what field to $in on. I'm assuming you want :

db.col.find({Id:{$in:[id1, id2, ..., idN]}})

Change your query construction code accordingly and you should be fine.

EDIT: Adding correct code :

public static void getDocuments(List<Integer> documentIds) {

            BasicDBList docIds = new BasicDBList();
            docIds.addAll(documentIds)
            DBObject inClause = new BasicDBObject("$in", docIds);
            DBObject query = new BasicDBObject("Id", inClause);
            DBCursor dbCursor = mongoRule.getDatabase().getCollection("mycollection").find(query);
            System.out.println(dbCursor == null);
            if (dbCursor != null) {
                while (dbCursor.hasNext()) {
                    System.out.println("object -  " + dbCursor.next());
                }
            }
        }

Please note that this assumes "Id" is something other than "_id"

Problem

I have a `JUnit rule` called as `MongoRule` looks like ``` public class MongoRule extends ExternalResource { private static final Logger LOGGER = LoggerFactory.getLogger(MongoRule.class); private final MongoService mongoService; public MongoRule() throws UnknownHostException { mongoService = new MongoService(getConfiguredHost(), getConfiguredPort(), getConfiguredDatabase()); } @Override protected void before() throws Throwable { LOGGER.info(" Setting up Mongo Database - " + getConfiguredDatabase()); } @Override protected void after() { LOGGER.info("Shutting down the Mongo Database - " + getConfiguredDatabase()); mongoService.getMongo().dropDatabase(getConfiguredDatabase()); } @Nonnull public DB getDatabase() { return mongoService.getMongo().getDB(getConfiguredDatabase()); } @Nonnull public Mongo getMongo() { return mongoService.getMongo(); } @Nonnull public MongoService getMongoService() { return mongoService; } public static int getConfiguredPort() { return Integer.parseInt(System.getProperty("com.db.port", "27017")); } @Nonnull public static String getConfiguredDatabase() { return System.getProperty("com.db.database", "database"); } @Nonnull public static String getConfiguredHost() { return System.getProperty("com.db.host", "127.0.0.1"); } } ``` Then I try to insert some documents as following ``` public static void saveInDatabase() { LOGGER.info("preparing database - saving some documents"); mongoRule.getMongoService().putDocument(document1); mongoRule.getMongoService().putDocument(document2); } ``` Where `document1` and `document2` are valid `DBObject` documents. The schema looks like ``` { Id: 001 date_created: 2012-10-31 vars: { '1': { name: n1 value:v1 } '2': { name: n2 value:v2 } '3': { name: n3 value:v3 } } { Id: 002 date_created: 2012-10-30 vars: { '1': { name: n4 value:v4 } '2': { name: n5 value:v5 } '3': { name: n6 value:v6 } } ``` Now I try to query the collection and get these objects, so I do this ``` public static void getDocuments(List<String> documentIds) { BasicDBList docIds = new BasicDBList(); for (String docId: documentIds) { docIds.add(new BasicDBObject().put("Id", docId)); } DBObject query = new BasicDBObject(); query.put("$in", docIds); DBCursor dbCursor = mongoRule.getDatabase().getCollection("mycollection").find(query); System.out.println(dbCursor == null); if (dbCursor != null) { while (dbCursor.hasNext()) { System.out.println("object - " + dbCursor.next()); } } } ``` `mycollection` is the collection where all documents are persisted, this comes from an external service. When I run this document I see following ``` preparing database - saving some documents inserting document - DBProposal # document1 inserting document - DBProposal # document2 false ``` Which means `collection.find()` could not find these documents. What is that I am not doing right here? How can I get the documents back? I am very new to using `Java` with `Mongo` and used this reference to construct the query UPDATE After changing the way query is constructed, I still don't see documents ``` public static void getDocuments(List<String> documentIds) { BasicDBList docIds = new BasicDBList(); docIds.addAll(documentIds) DBObject query = new BasicDBObject(); query.put("$in", docIds); DBCursor dbCursor = mongoRule.getDatabase().getCollection("mycollection").find(query); System.out.println(dbCursor == null); if (dbCursor != null) { while (dbCursor.hasNext()) { System.out.println("object - " + dbCursor.next()); } } } ``` and the collection Name is returned via ``` private static String getCollectionName(@Nonnull final DBObject dbObject) { return "mycollection"; } ```

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