> db.people.save(

{ fname: "BOB", lname: "SMITH" }

)
> db.people.find(

{ lname: "SMITH" }

)

{ "_id" : ObjectId("4c06e2eb4138a24d33ee0fdc"), "fname" : "BOB", "lname" : "SMITH" }

> db.people.save(

{ fname: "BOB", lname: "SMITH", mname: "ROBERT", suffix: "JR" }

)
> db.people.ensureIndex(

{"lname":1,"fname":1,"mname":1,"suffix":1}

);
> db.people.find({ lname: "SMITH", mname:

{ $exists: false }

})

-Nothing Returned-

Also, using null for the value retrieves the doc even thought that is incorrect (since the index stores a null for that index value).

> db.people.find(

{ lname: "SMITH", mname: null }

)

{ "_id" : ObjectId("4c06e2eb4138a24d33ee0fdc"), "fname" : "BOB", "lname" : "SMITH" }