Select distinct values from MongoDB array using Java

Question

I want to get a list of unique values from a List field called categories.

I am using Java and MongoDB. Most of the examples and docs I can see seem to suggest I need to do something like what I have below:

public static List<String> listCategories(String input) {

    Datastore ds = Dao.instance().getDatabase();
    BasicDBObject dbObject=new BasicDBObject("categories", input);
    DBCollection dBCollection = ds.getCollection(Product.class);
    List<String> categories = dBCollection.distinct("categories",dbObject);

    return categories;
}

However when I test it using this code:

@Test
public void testListCategories(){
    List<String> categories = Product.listCategories("S");
    Assert.assertTrue(categories.size() > 0);
}

The test fails even though I know there are categories that start with S (have also tried a few others just to be sure).

Is it even possible to do this, if so do you have any pointers?

Thanks

BDRSuite · Accepted Answer · 2014-12-04 17:23:33Z

1

BasicDBObject dbObject=new BasicDBObject();
dbObject.append(categories", input);

Now it will be like a where condition,

DBCollection dBCollection = ds.getCollection(Product.class);
 List<String> categories = dBCollection.distinct("categories",dbObject);

answered Dec 4, 2014 at 17:23

BDRSuite

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BatScream · Accepted Answer · 2014-12-04 18:35:36Z

1

The test fails even though I know there are categories that start with S (have also tried a few others just to be sure).

You need to perform a regex match using the $regex operator on the categories field to find distinct categories that start with a particular input.

Your updated code should look like:

BasicDBObject like = new BasicDBObject("$regex",
                         java.util.regex.Pattern.compile("^"+input,
                                                      Pattern.CASE_INSENSITIVE));
BasicDBObject query = new BasicDBObject("categories", like);
List<String> categories = dBCollection.distinct("categories",query);

answered Dec 4, 2014 at 18:35

BatScream

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