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In a project I'm working on, at one point I read a query to mongodb from a string. I've been using com.mongodb.util.JSON.parse(querystring) to read the query, which worked fine until I started reading queries that contained operators like $max and $min. At that point, rather than using mongodb's $max operator, the parser instead creates a "$max" field. For instance,

the input string:

{ $query : { state : "AL" } , $max : { pop : 9058 } }

is parsed to the DBObject:

{ "$query" : { "state" : "AL"} , "$max" : { "pop" : 9058}}

When I then look for a DBCursor with that query document, I get a cursor of size 0 (no matching document found in the databse), presumably because there are no documents with "$query" or "$max" fields.

Is there something I can use besides JSON.parse()? I'm not averse to writing my own function for it, but how can I get a DBObject that recognizes the $ operators as operators and not fields?

Any advice would be appreciated!

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2 Answers

up vote 0 down vote

The following code snippet using query modification operator $max seems to work fine.

/* {$query:{state:"AL"}, "$max":{pop:10000}}*/
String s = "{$query:{state:\"AL\"}, \"$max\":{pop:10000}}";
DBObject dbObject = (DBObject) JSON.parse(s);
System.out.println("\nFind all: ");

DBCursor cursor = collection.find(dbObject);

try {
    while (cursor.hasNext()) {
        DBObject cur = cursor.next();
        System.out.println(cur);
    }
} finally {
   cursor.close();
}

Make sure you have specified index on pop.

db.zips.getIndexes()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"ns" : "test.zips",
"name" : "id"
},
{
"v" : 1,
"key" : {
"pop" : 1
},
"ns" : "test.zips",
"name" : "pop_1"
}
]

See the following link for detail.
http://docs.mongodb.org/manual/reference/operator/max/
Just in case you are interested in using aggregation operators $max or $min, the following link provide details and sample code.
http://docs.mongodb.org/ecosystem/tutorial/use-aggregation-framework-with-java-driver/

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That does in fact work, though I wound up with a different solution. Thanks, though! –  firechant Jun 27 '13 at 21:01
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up vote 0 down vote accepted

So it turns out the DBObject as given up there worked out fine. It returns a cursor with a size of 0, true, but the DBCursor's length is actually the thing I was looking for. (Previously, I had been checking whether the cursor's size was 0, and if it was, returning null.)

I'm not quite sure what the difference between size and length is in a DBCursor (the difference between size and count is apparent, but I'm not sure what length is supposed to be), but it works now. In the case above, size and count were both 0 but length was the desired number.

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