I am developing the application in which system will show the campaign content to users which he/s never seen. To keep the logs either the specific user seen the specific content or not, i am maintaining logs of user activity in MongoDB collection. Here's what a campaign record in the database looks like.

campaigns

{
    "campaign_id": "123",
    "campaign_image": "<image path>",
    "min_user_age": 10
}

Here is how the user's activity log collection like

user_activity

{
    "i123i12391i2391i23kjnsa": {
        "user_id": 12,
        "campaign_ad_id": 34234,
    },
    "xajeou424u2934y293y4293": {
        "user_id": 32423,
        "campaign_ad_id": 34234,
    },
    ...
}

The user_activity collection will have approx 7 million records and campaigns will have more than 10 thousand records. I will always have user's id to get the campaign to show him. Here i want to get only those campaigns from campaigns collection which are not seen by the specific user.

I can do like fetch records from campaigns and then check that their records do not exist in user_activity collection based on specific user_id.

$campaigns = db.getCollection('campaigns').find({});

Loop though $campaigns and make sure activity does not exist for particular campaign_id and user_id.

$isExists = db.getCollection('user_activity').find({"campaign_id": 34234, "user_id": 12});

What is the best way to get the campaigns that are not seen by the user?

Thanks