Push an element to an array if it is not present (no duplicates)

Question

I have an event collection which I'm going through to find a category to the specific event and then I'm updating my other collection using a $push statement. The problem is that when two events have the same category it'll create a duplicate, which I don't want. I know about upserts but I am unsure if they are the best way to go regarding this? And I'm a bit confused when it comes to how to actually write an upsert that works with the "$push"-statement. This is how my update looks right now: ``` self.users.update({"user_id": event['userid']}, {'$push': {'campaigns': UserCampaign}}) ``` ..where: ``` UserCampaign = { "id": campaign['id'], "name": campaign['name'] } ``` The "UserCampaign" gets filled up with the same information from time to time, and since my collection is probably gonna be very huge I want to complete this as efficient as possible. TLDR; I want to update the array in the document found using a "push" without having the risk of getting duplicates.

Accepted Answer

Found a better answer to my problem:

By using $addToSet it didn't create duplicates (I also made sure no duplicates where made before by adding all dictionaries to a list):

```
self.users.update({"user_id": event['userid']}, {'$addToSet': {'campaigns': UserCampaigns[i]}})
```

If I just had used $push it would always create duplicate elements in 'campaigns' within the users collection. This happened with and without upsert.

For some reason $each didn't work but wasn't required, I guess PyMongo takes care of that for me.

Push an element to an array if it is not present (no duplicates)

Solution

Problem