How do I learn the fields/keys of a collection?

[Sky USC]

I am new. please help.
 
I roughly have a collection with approx 5 million rows. The schema looks like this:
 
* ID : Folder.1108
* location : memphis
* DocType :
   * "jpg" : 53    
   * "gif"  : 41
 
* ID : Folder.1109
* location : memphis
* DocType :
* "pdf" : 15    
* "jpg" : 3
 
* ID : Folder.1110
* location : portland
* DocType :
   * "pdf" : 39    
   * "gif" : 21
   * "jar" : 6
     etc etc.. and there could be unknown types of document extensions.
 
I am trying to find the command to
1. List - what are the distinct names of file extensions possibly found in all of the documents. Can this be done without iterating thru each of the documents? Also, hopefully with ability to find extensions that are only in the "memphis" location.
2. Possibly find the actual count of "jpg" in memphis location alone.
 
Thx.
 

[Scott Hernandez]

I'm afraid you need to go through them all. You can limit the ones you
need to go through with a query including the location.

> 2. Possibly find the actual count of "jpg" in memphis location alone.

You could change your structure to this:

{ ID : "Folder.1109", location : "memphis", DocType : [ {type:"pdf" :
count:15}, {type:"jpg" : count:3}]}
to be able to find({location:"memphis", "DocType.type":"pdf"})

using this index: {location:1, "DocType.type": 1}

> Thx.
>
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[Sky USC]

Thx. The "Type"/"Count" suggestion works well!

[Scott Hernandez]

If you are storing lots of those you may want to just use t/c instead
of the whole words.