I think in the context of purchases @arun-awnics, you were right the first time. Documents in mongodb have a 16MB hard limit. Imagine if amazon used mongodb to store a user's purchases in an array of embedded docs. After roughly 10 years of saved purchase info, I'm pretty sure my user would be broken on Amazon, or they'd have to jump through some serious hoops to store all of my historical info. It makes more sense to me to have a purchases collection with a field in the docs that points back to the user.