> db.not.insert(

{a:1}

)
> db.not.insert(

{a:2}

)
> db.not.insert({})
> db.not.find({})

{"_id" : ObjectId( "4a70d57450cc2b64ff07f162") , "a" : 1} {"_id" : ObjectId( "4a70d57650cc2b64ff07f163") , "a" : 2} {"_id" : ObjectId( "4a70d57850cc2b64ff07f164") }

> db.not.find(

{a:1}

)

{"_id" : ObjectId( "4a70d57450cc2b64ff07f162") , "a" : 1}

> db.not.find({a:{$ne:1}})

{"_id" : ObjectId( "4a70d57650cc2b64ff07f163") , "a" : 2}

> db.not.find({a:{$in:[1]}})

{"_id" : ObjectId( "4a70d57450cc2b64ff07f162") , "a" : 1}

> db.not.find({a:{$nin:[1]}})

{"_id" : ObjectId( "4a70d57650cc2b64ff07f163") , "a" : 2}

Note that the empty document is never found. This is surprising if you are expecting $nin to return all docs in the collection not returned by $in (same for $ne and $eq).