How can I build an $or query for MongoDB using the Java driver?

Question

I'm trying to Or some conditions in MongoDB (using the Java driver). This is what I'm doing :

Pattern regex = Pattern.compile("title");   
DBCollection coll = MongoDBUtil.getDB().getCollection("post_details");

BasicDBObject query = new BasicDBObject();
query.put("category_title", "myCategory");      
query.append("post_title", regex);  
query.append("post_description", regex);    

DBCursor cur = coll.find(query);
while(cur.hasNext()) {
    System.out.println(cur.next().get("post_id"));
}

I'd like to use the $or operand on these conditions, but I guess the default is "and" and I don't know how to change it. In the above code if one of the conditions returns null, the result will be null too.

Answer 1

You are correct that the "default" for specifying multiple field in a query is that each field serves as a conditional filter, and thus is an AND operation.

You can perform MongoDB queries with an OR clause by using the $or operand which has the following syntax :

db.col.find({$or:[clause1, clause2]})

Where each clause can be a query. $or does not have to be a top level operand but if it is MongoDB can use an index for each seperate clause.

In your example you want to end up with this query :

db.col.find({$or:[{"post_title", regex}, {"post_description", regex}]});  

Which can be constructed in Java through :

DBObject clause1 = new BasicDBObject("post_title", regex);  
DBObject clause2 = new BasicDBObject("post_description", regex);    
BasicDBList or = new BasicDBList();
or.add(clause1);
or.add(clause2);
DBObject query = new BasicDBObject("$or", or);