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 it seems like I have searched everywhere..

thanks

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 } }
)