How do I handle big arrays in MongoDB?

Question

I have a user document, and it has an array field called activityLog. Every action the user takes needs to be recorded here. However, I know MongoDB caps document sizes at 16MB. If this array needs to be able to hold potentially millions of items, won't that exceed the document size limit? How can I handle that?

Please answer this generally, ie some generic array needs to hold potentially millions of items

Answer 1

From the use case you have described, it looks like activity logs belong to a separate collection. As you already mentioned you can not embed logs to a user because the data grows a large amount and can exceed the 16 MB document size, so using references is the way to go. Next, you have to decide whether to reference from a user to logs or from logs to a user (parent-referencing). Having an array of references to logs in a user is not as good as parent-referencing, because it still can overflow the document size limit, also you will hardly need all user history each time you retrieve a user.