When asking for just an array inside a dict, only that array is returned (good so far). However, when you ask for a slice of that array, other items in that dict are also returned.

> db.a.save({_id:1, vals: {first:[1,2,3,], second:[1,2,3]}})
> db.a.find()
{ "_id" : 1, "vals" :

{ "first" : [ 1, 2, 3 ], "second" : [ 1, 2, 3 ] }

}
> db.a.find({},{"vals.first":{$slice:2}})
{ "_id" : 1, "vals" :

{ "first" : [ 1, 2 ], "second" : [ 1, 2, 3 ] }

}

//I would have expected this to work... might call that a bug.

> db.a.find({},{"vals.first":1, "vals.first":{$slice:2}})
{ "_id" : 1, "vals" :

{ "first" : [ 1, 2 ], "second" : [ 1, 2, 3 ] }

}

> db.a.find({},{"vals":0, "vals.first":{$slice:2}})
{ "_id" : 1, "vals" :

{ "first" : [ 1, 2 ] }

}

(thanks skot on the #mongodb IRC channel for the example)