MongoDB - find and findOne with nested array filtering. Finally!

What if we could filter the array only, but instead of returning the filtered array as the result, use it to override the original one? Turns out we can:

db.getCollection('posts').aggregate(
    {$match: {"author.id": authorId}},
	{$addFields : {"comments":{$filter:{ // We override the existing field!
		input: "$comments",
		as: "comment",
		cond: {$eq: ["$$comment.state.deleted", false]}
	}}}}
);

That's it. This little trick allows nested array filtering. I also suspect that I could patch it for nested nested array filtering, but that's for another post.

B.t.w the results:

{
  "author": {},
  "message": "This is post1",
  "comments": [
    {
      "message": "Im number 1!!!",
      "state": {
        "deleted": false
      }
    },
    {
      "message": "tHIS IS GREAT!",
      "state": {
        "deleted": false
      }
    },
    {
      "message": "I can type better than you guys",
      "state": {
        "deleted": false
      }
    }
  ]
},
{
  "author": {},
  "message": "This is post 2",
  "comments": [
    {
      "message": "I wanna have your children",
      "state": {
        "deleted": false
      }
    }
  ]
}