MongoDB – Save Image as Bytes or Files

Before you go further you need to answer a few questions

• how do you represent files within folders into the database
• how do you represent folders
• do you have relations between folders (parent -> child)
• how often do you expect to create folders and files
• how often do you update existing files into folders and what is the number of files you update

Based on your answers you can have a write optimized schema or a read optimized schema. Write optimized is a schema that contains many entries that are very small or you can use built in operators like $inc over a collection. Read optimized is generally a larger collection like the one you described, into your scenario you could have very easy something like this (assuming all folders are at the same level)

{ "userid" : "email or id",
  [ 
     { "folder1" : [ "file1", "file2"] },  
     { "folder2" : [ "file3", "file4"] },
  ]
}

But with this schema it gets quite complicated if you need to link a folder to a parent folder ... But is obvious that the userid is the shard key.

Logging is always a good use case for non relational stores (text book example).

You've only given us one query pattern, so going by that I would create a single compound index, equality to inequality.

I would suggest reading up on indexes at:

MongoDB Index Introduction

and

MongoDB FAQ: Indexes