Using Mongoose / MongoDB $addToSet functionality on array of objects

Question

say I have this array property ('articles') on a Mongoose schema: ``` articles: [ { kind: 'bear', hashtag: 'foo' }, { kind: 'llama', hashtag: 'baz', }, { kind: 'sheep', hashtag: 'bar', } ] ``` how can I use $addToSet https://docs.mongodb.org/manual/reference/operator/update/addToSet/ to add to this array by checking the value of hashtag to see if it's unique? For example, if I want to add the following object to the above array, I want Mongo to 'reject' it as a duplicate: ``` { kind: 'tortoise', hashtag: 'foo' } ``` because hashtag=foo has already been taken. The problem is that I only know how to use `$addToSet` with simple arrays of integers... for example, if articles looked like this: ``` articles: [ 1 , 5 , 4, 2] ``` I would use $addToSet like this: ``` var data = { "$addToSet": { "articles": 9 } } model.update(data); ``` but how can I accomplish the same thing with an array of objects where the unique field is a string, in this case 'hashtag'? The docs don't make this clear and…

Accepted Answer

You need to use the `$ne` operator.

```
var data = { 'kind': 'tortoise', 'hashtag': 'foo' };
Model.update(
    { 'articles.hashtag': { '$ne': 'foo' } }, 
    { '$addToSet': { 'articles': data } }
)
```

This will update the document only if there is no sub document in the "article" array with the value of hashtag equals to "foo".

As @BlakesSeven mentioned in the comment

The `$addToSet` becomes irrelevant once you are testing for the presence of one of the values, so this may as well be a `$push` for code clarity. But the principle is correct since `$addToSet` works on the whole object and not just part of it.

```
Model.update({ 
    { 'articles.hashtag': { '$ne': 'foo' } }, 
    { '$push': {'articles':  data } }
)
```

Using Mongoose / MongoDB $addToSet functionality on array of objects

Solution

Problem