Using QueryBuilder to find an array that contains an element

Question

I have a database structure like this:

{
    "_id" : "Foo",
    "registrations" : [ 
        101, 
        102
    ]
}

What I would like to do is create a query that will find all documents that contain a "registrations" array with the value 101 contained within it. I would like to use the QueryBuilder if at all possible because I typically find it easier to use.

Based on this answer, I tried to use elemMatch() to find it, but that didn't seem to have the desired effect:

QueryBuilder.start().put("registrations").elemMatch(new BasicDBObject("_id", 101)).get();

However, the stacktrace said that it is trying to cast the BasicDBObject to an Integer and failing.

java.lang.ClassCastException: java.lang.Integer cannot be cast to com.mongodb.DBObject

So how can I create a query using QueryBuilder that will find all documents that contain a "registrations" array witht he value 101 contained within it?

Thunderforge · Accepted Answer · 2015-05-29 18:15:59Z

0

After a bit more experimenting, I discovered that what I am wanting is the $in operator, which allows you to check whether or not an array contains particular values. For my specific query, the codeI was looking for was:

QueryBuilder.start().put(registrations).in(Arrays.asList(101)).get();

This returns the DBObject with id "Foo".

If I were looking for more than one registration, I could put them in the list that is the parameter for in().

answered May 29, 2015 at 18:15

Thunderforge

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