Best Practice for Monitoring document changes

Just wanted to ask about the best-practice for keeping track of changes of a document. E.g. a document is a blog post and users can "like" it. Currently I am saving the userIds in an array to keep track, who is liking it. But now I want to create an Activity-View to tell other users "user xy liked the post xy". My plan was to create a function which is triggered when the blog post document is changed, and create the right Activity-documents for the other users on the server-side. But I can't check what is changed, since it could be anything.

So now I am stuck between a few options:

• change the like-array into something more complex (e.g. array with id;timestamp;isNew-Bool)
• add a flag-attribute (e.g. an last-update-type = "newLike")
• Create the Activites when the user is pressing the like-button (which I dont want, since its more load on the client-side)
• separate collection for likes (same argument as above, additional load on client-side)
• ...

But I hope there is a more simpler approach which I am missing right know. Thank you.