Can MongoDB use an index when checking for existence of a field with $exists operator?

Question

If I have data in my users collection that looks like:

{ name: '...',    email: '...',    ...,   photos: {      123: { url: '...', title: '...', ... },      456: { url: '...', title: '...', ... },      ...   } }  

And I want to find which user owns photo id 127, then I am using the query:

db.users.find( {'photos.127': {'$exists' => true} } ); 

I've tried, but it doesn't seem possible to get MongoDB to use an index for this query. The index I tried was: db.users.ensureIndex({photos:1});. And when I used explain() mongo told me it was using a BasicCursor (i.e., no index was used).

Is it possible to create an index that mongo will use for this query?

Answer 1

Updated:

Seems $exists queries use index properly now based on these tickets $exists queries should use index & {$exists: false} will not use index

Old Answer:

No, there is no way to tell mongodb to use index for exists query. Indexing is completely related to data. Since $exists is only related to the keys (fields) it cant be used in indexes.

$exists just verifies whether the given key (or field) exists in the document.

Answer 2

Since MongoDB 2.0 $exists queries should use an index. Unfortunately this fix has disappeared in the newest version and will be fixed in MongoDB 2.5