Max size of MongoDB array when using $addToSet

Question

using:

 db.collection('users').update(
     { "_id": user._id},
     { "$addToSet": { "keywords.RFD": keywords } },
     function(err, result) {
         if (err) {
             console.log(
               'failed to add keyword "%s" for user: %s', keywords, user.email);
                    throw err;
          }
          else {
              console.log('Keyword added: ' + keywords );
              res.end('{"success" : "Updated Successfully.", "status" : 200}');
          }
   }
);

I would like to limit the number of keywords to 50 and throw an error if the keywords exceed 50. Is there anyway I can check with MongoDB if there are already 50 in the database?

Answer 1

I thought maybe there was a way to add a strict limit to the array itself using MongoDB like a capped array but I don't think that is possible. I ended up just using logic.

if (user.settings.max_keywords <= result.keywords.RFD.length){
        res.end('{"error" : "Too many keywords.", "status" : 401}');
      }

Answer 2

const MAX_KEYWORDS = 50;
const maxArrayPath = `keywords.RFD.${MAX_KEYWORDS - 1}`;

db.collection('users').updateOne(
  { _id: user._id,
    [maxArrayPath]: { $exists: false }
  },
  { 
    $addToSet: { 
      "keywords.RFD": keywords,
    }
  }
);

Answer 3

Tell you what. You didn't read the documentation properly. So keywords is a "misnomer" here as it can only be a singular string or other object in this context. Certainly not an array.

But yes MongoDB has a way to handle this. Much as I gave the pointer, you need to read the $each documentation, along with other modifiers:

db.collection('users').update(
  { "_id": user._id},
  { 
    "$addToSet": { 
      "keywords.RFD": {
         "$each": [keywords],
      }
  }
)

Then you can actually invoke a separate update statement invoking $slice with a blank call to $each. Which, funnily enough, works perfectly.

db.collection('users').update(
  { "_id": user._id},
  { 
    "$push": { 
      "keywords.RFD": {
         "push": {"$each": [] },
      },
      "$slice": -50
  }
)

The main key modifier here is $slice which is a form to "limit" the size of an array.

Take note that you need MongoDB 2.6 or greater as a server version for these operators to be valid on updates. It's a compelling reason to upgrade though if you need this sort of interaction.

P.S.: As I said earlier, the context of "keywords" in your example was a "misnomer" beacause it hat to be a "scalar" value. Under the $each modifier, this can indeed be an array. Therefore:

db.collection('users').update(
  { "_id": user._id},
  { 
    "$addToSet": { 
      "keywords.RFD": {
         "$each": { "$each": keywords },
      }
  }
)
db.collection('users').update(
  { "_id": user._id},
  { 
    "$push": { 
      "keywords.RFD": {
         "push": [],
      },
      "$slice": -50
  }
)

Not atomic. But as close as you can get, especially when using the Bulk API.