> db.foo.find()

{ "_id" : ObjectId("4ca0200bfe2f0f60296390db"), "a" : 2 } { "_id" : ObjectId("4ca0200ffe2f0f60296390dc"), "a" : "3" } { "_id" : ObjectId("4ca0204afe2f0f60296390dd"), "a" : [ 1, 2, 3, 4 ] } { "_id" : ObjectId("4ca02058fe2f0f60296390de"), "a" : [ "a", "b" ] }

Wrong:
> db.foo.find({a : {'$type' : 1}})

{ "_id" : ObjectId("4ca0200bfe2f0f60296390db"), "a" : 2 } { "_id" : ObjectId("4ca0204afe2f0f60296390dd"), "a" : [ 1, 2, 3, 4 ] }

(should not return the 2nd hit)

Wrong:
> db.foo.find({a : {'$type' : 2}})

{ "_id" : ObjectId("4ca0200ffe2f0f60296390dc"), "a" : "3" } { "_id" : ObjectId("4ca02058fe2f0f60296390de"), "a" : [ "a", "b" ] }

(should not return the 2nd hit)

Wrong:
> db.foo.find({a : {'$type' : 4}})

Returns nothing but should return the rows with the arrays.