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

Question

I would like to execute a following query:

db.mycollection.find(HAS IMAGE URL)

What should be the correct syntax?

Answer 1

db.collection_name.find({"filed_name":{$exists:true}});

fetch documents that contain this filed_name even it is null.

Warning

db.collection_name.find({"filed_name":{$ne:null}});

fetch documents that its field_name has a value $ne to null but this value could be an empty string also.

My proposition:

db.collection_name.find({ "field_name":{$ne:null},$where:"this.field_name.length >0"})