Casted type-change updates are discarded

A simple example: // insert a document into a new table; number is interpreted as a double db.example.insert({"foo": 1}) //--> WriteResult({ "nInserted" : 1 })   // good so far db.example.find() //--> { "_id" : ObjectId("56b4c6dcbfa50a533004158a"), "foo" : 1 }   // confirm the field is of type double // ref: https://docs.mongodb.org/manual/reference/operator/query/type db.example.count({"foo": {"$type": 1}}) //--> 1   // attempt to update field from double to integer; fails db.example.update({"foo": {"$type": 1}}, {"$set": {"foo": NumberInt(1) }}) //--> WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 })   // doc still has a double db.example.count({"foo": {"$type": 1}}) //--> 1   // update field from double to integer of a different value; succeeds db.example.update({"foo": {"$type": 1}}, {"$set": {"foo": NumberInt(2) }}) // WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })   // doc no longer has a double db.example.count({"foo": {"$type": 1}}) //--> 0   // confirm doc now has an int db.example.count({"foo": {"$type": 16}}) //--> 1