mongodb is so good

[梵总]

We can update 'array' filed with a unique value with "$addToSet". How amazing is it? I used to get the field value, check the new data is in the array or not, and save the array back.

urls = [j.url for j in a.followers]
db.graph.update_one({"user_id": a.url}, {"$addToSet": {"following": {"$each": urls}}})

The best way to do this is using $addToSet operator which ensures that there are no duplicate items added to the set and the $each modifier to add multiple values to the"following" array.

REF: https://stackoverflow.com/questions/36518635/avoid-a-duplicate-value-when-i-update-array-using-push-on-mongodb

[梵总]

Also, the index is super important in MongoDB. Don't forget you can create an index on a field that will be created.