MongoDb query array with null values

Question

I have objects in my collection that look like:

{ MyArray:[null, \"some value\", null] }

I want to query those objects that have a null value a

Answer 1

In order to find documents with arrays, with null elements please run:

db.collection.find({"keyWithArray":{$elemMatch:{"$in":[null], "$exists":true}}})

According to safaribooksonline proper null matching is performed using $in (because you cannot use $eq with null). Also, comparing with null:

{"something":null}

will match documents with "something" field set to null and every document which does not have "something" field at all. Thus we have to make sure the key exists using $exists.