How do you query for “is not null” in Mongo?

I would like to execute a following query:

db.mycollection.find(HAS IMAGE URL)

What should be the correct syntax?

This will return all documents with a key called “IMAGE URL”, but they may still have a null value.

db.mycollection.find({"IMAGE URL":{$exists:true}});

This will return all documents with both a key called “IMAGE URL” and a non-null value.

db.mycollection.find({"IMAGE URL":{$ne:null}});

Also, according to the docs, $exists currently can’t use an index, but $ne can.

Edit: Adding some examples due to interest in this answer

Given these inserts:

db.test.insert({"num":1, "check":"check value"});
db.test.insert({"num":2, "check":null});
db.test.insert({"num":3});

This will return all three documents:

db.test.find();

This will return the first and second documents only:

db.test.find({"check":{$exists:true}});

This will return the first document only:

db.test.find({"check":{$ne:null}});

This will return the second and third documents only:

db.test.find({"check":null})