title: How to call $addToSet if an array exists if it doesnt then call $set in one query?
body: <p>Hey so I am trying to set a new array that does not exist in the mongodb docs, but I want to be able to throw additional documents into the array without constantly replacing the values.</p>
  
  <p>for example:</p>
  
  <pre><code>users {
      id: 'value1',
      name: {value2},
      friends: [
                 {friendID: 'value x', type: 'valuey', TAGS: [tag1, tag2]}
  
               ]
  </code></pre>
  
  <p>Basically TAGS does not exist and I want to create it and add tag1, and if it does exist I want to just add tag2 to the array...</p>
  
  <p>This is to make sense of the variables in the sample above to what I have in my db:</p>
  
  <pre><code>friendStatus = friends,
  fuId = friendID,
  labels = TAGS
  </code></pre>
  
  <p>Here is my attempt:</p>
  
  <pre><code>Friend.findOneAndUpdate({userId: mongoose.Types.ObjectId(req.signedCookies.userid), 
  'friendStatus.fuId': mongoose.Types.ObjectId(req.body.user)}, 
  {$addToSet: {'friendStatus.$.labels': req.body.labelNew}}
      , function(err, userX) {    
  
  
  }
  </code></pre>