example output

findAndReplace('games.$._id', obj, function (key, value) {
  return Collections['games'].findOne(value);
});

// expected result:

{
  "battletag": "corvid#1376",
  "games": [{
    "overwatch": {
      "characters": [{
        "_id": "1234",
        "class": "Hanzo",
        "league": "Diamond"
      }, {
        "_id": "5678",
        "class": "Widowmaker",
        "league": "Diamond"
      }]
    }
  }, {
    "world of warcraft": {
      "characters": [{
        "_id": "234",
        "name": "Corvid",
        "class": "Shaman",
        "league": "Bronze"
      }]
    }
  }]
}

example.json

{
  "battletag": "corvid#1376",
  "games": [{
    "overwatch": {
      "characters": [{
        "_id": "1234",
      }, {
        "_id": "5678",
      }]
    }
  }, {
    "world of warcraft": {
      "characters": [{
        "_id": "234",
      }]
    }
  }]
}