MongoDB conditionally $addToSet sub-document in array by specific field

Question

Is there a way to conditionally $addToSet based on a specific key field in a subdocument on an array?

Here's an example of what I mean - given the collection produced by the following sample bootstrap;

cls
db.so.remove();
db.so.insert({
    "Name": "fruitBowl",
    "pfms" : [
        {
            "n" : "apples"
        }
    ]
});

n defines a unique document key. I only want one entry with the same n value in the array at any one time. So I want to be able to update the pfms array using n so that I end up with just this;

{
    "Name": "fruitBowl",
    "pfms" : [
        {
            "n" : "apples",
            "mState": 1111234
        }
    ]
}

Here's where I am at the moment;

db.so.update({
  "Name": "fruitBowl",
},{
// not allowed to do this of course
//    "$pull": {
//  "pfms": { n: "apples" },
//    },

"$addToSet": {
  "pfms": {
    "$each": [
      {
        "n": "apples",
        "mState": 1111234
      }
    ]
   }
  }
 }
)

Unfortunately, this adds another array element;

db.so.find().toArray();

[
    {
        "Name" : "fruitBowl",
        "_id" : ObjectId("53ecfef5baca2b1079b0f97c"),
        "pfms" : [
            {
                "n" : "apples"
            },
            {
                "n" : "apples",
                "mState" : 1111234
            }
        ]
    }
]

I need to effectively upsert the apples document matching on n as the unique identifier and just set mState whether or not an entry already exists. It's a shame I can't do a $pull and $addToSet in the same document (I tried).

What I really need here is dictionary semantics, but that's not an option right now, nor is breaking out the document - can anyone come up with another way?

FWIW - the existing format is a result of language/driver serialization, I didn't choose it exactly.

further

I've gotten a little further in the case where I know the array element already exists I can do this;

db.so.update({
    "Name": "fruitBowl",
    "pfms.n": "apples",
},{
  $set: {
   "pfms.$.mState": 1111234,
  },
 }
)

But of course that only works;

1. for a single array element
2. as long as I know it exists

The first limitation isn't a disaster, but if I can't effectively upsert or combine $addToSet with the previous $set (which of course I can't) then it the only workarounds I can think of for now mean two DB round-trips.

Answer 1

I have faced the exact same scenario. I was inserting and removing likes from a post.

What I did is, using mongoose findOneAndUpdate function (which is similar to update or findAndModify function in mongodb).

The key concept is

• Insert when the field is not present
• Delete when the field is present

The insert is

findOneAndUpdate({ _id: theId, 'likes.userId': { $ne: theUserId }},
{ $push: { likes: { userId: theUserId, createdAt: new Date() }}},
{ 'new': true }, function(err, post) { // do the needful }); 

The delete is

findOneAndUpdate({ _id: theId, 'likes.userId': theUserId},
{ $pull: { likes: { userId: theUserId }}},
{ 'new': true }, function(err, post) { // do the needful }); 

This makes the whole operation atomic and there are no duplicates with respect to the userId field.

I hope this helpes. If you have any query, feel free to ask.