title: PyMongo $addToSet not working
body: <p>So i am using <code>pymongo==3.2.1</code> and <code>Mongo 3.2</code> but for some reason my <code>$addToSet</code> (to avoid duplicates) is not working.</p>
  
  <p>I made sure my <code>checked_on</code> timestamp matched like for example <code>ISODate("2016-04-21T00:00:00.000Z")</code></p>
  
  <p>My structure is like this:</p>
  
  <pre><code>{
      "_id": ObjectID("57062e357e254255a531165e"),
      "availability": [
          {
              "checked_on": ISODate("2016-04-21T00:00:00.000Z"),
              "days": [
                  {
                      "available": false,
                      "day": ISODate("2016-04-29T00:00:00.000Z")
                  },
                  {
                      "available": false,
                      "day": ISODate("2016-04-30T00:00:00.000Z")
                  }
              ]
          }
      ],
      "is_active": true,
      "updated_on": ISODate("2016-04-21T13:41:12.449Z")
  }
  </code></pre>
  
  <p>I have tried both:</p>
  
  <pre><code>result = self.db[self.collection_name].update_one(
      {'_id': doc['_id']},
      {
          '$addToSet': {
              'availability': {
                  '$each': [
                      {
                          'checked_on': datetime.datetime(
                              date.year, date.month, date.day
                          ),
                          'days': meta['availability']
                      }
                  ]
              }
          },
          '$set': {
              'is_active': is_active, 'updated_on': date
          }
      }
  )
  </code></pre>
  
  <p>and </p>
  
  <pre><code>result = self.db[self.collection_name].update_one(
      {'_id': doc['_id']},
      {
          '$addToSet': {
              'availability': {
                  'checked_on': datetime.datetime(
                      date.year, date.month, date.day
                  ),
                  'days': meta['availability']
              }
          },
          '$set': {
              'is_active': is_active, 'updated_on': date
          }
      }
  )
  </code></pre>
  
  <p>But duplicates keep appearing, what am i doing wrong?</p>