Perfomance of operations in a sharded cluster

I have a sharded collection of documents. Example of a single document:

  _id: {
    "a": "1",
    "b": "1",
    "c": "1"
  "someData": "abc"

Hashed shard key – “_id.a”

I have two main scenarios:

  • updating document;
  • querying documents.

Question №1: In first scenario i need to update a single document by _id. Will MongoDB treat this operation as targeted to a single shard?
Here i’m not providing an explicit sharding key, but according to a query plan this operation hits only one shard.

Question №2: In second scenario i would like to filter documents by _id using $in operator. All provided documents for $in operator have same value for field “_id.a”. Is it possible make MongoDB to treat this query as targeted to a single shard?
Of course i have a workaround to query by a shard key and then filter data in code, but i would like to get it by a single query in MongoDB.