When performing a query against indexed subdocument fields we are able to retrieve and update the subdocument within an array using the $ operator. There are cases where we would have need to know what subdocuments were located who satisfied our query and make use of that data.

A simple example would be to locate a document based on its subdocuments content and append a new subdocument that derives from that located subdocument...Given:

{ "SUBS" : [

{ "_id" : 1, "VALUE" : 5 }

], "_id" : 1 }

Would like to execute something like:

db.col.update(

{ "SUBS._id" : 1 }

, { $push : { "SUBS" :

{ "_id" : 3, "VALUE" : SUBS.$.VALUE * 5 }

} } )

That would result in appending a new subdocument "_id":3 in all documents who contain the matched subdocument from our query with a new value derived from the matched subdocument:

{ "SUBS" : [

{ "_id" : 1, "VALUE" : 5 }

,

{ "_id" : 3, "VALUE" : 25 }

], "_id" : 1 }

Please see (http://groups.google.com/group/mongodb-user/browse_thread/thread/7f0409356557aa64/468de81a1f938046?lnk=gst&q=serenityexperience#468de81a1f938046) for full discussion.

This relates to https://jira.mongodb.org/browse/SERVER-831, but in that case the subdocuments are being updated. This request is for the ability to access the subdocuments content who satisfied the query so we can make further decisions/modifications based on that content.

As an added advantage I could seen a lot of power added to JavaScripting if this were also available within JS, for example:

function( previousID, newID )
{
db.col.find(

{ "SUBS._id" : previousID }

).forEach(
function( thing )
{
previousValue = thing.SUBS.$.VALUE;
db.col.update(

{ "_id" : thing._id }

,
{ $push : { "SUBS" :

{ "_id" : newID, "VALUE" : previousValue * 5 }

} } );
} );
}

In this case, where the primary document was queried based on its subdocument contents, it would be significant if we had access to that data through the passed in matched Document. I am not sure if this would be the ideal syntax but having some way to reference the specific subdocument that satisfied the this outer query can give a lot of power to the inner for making decisions based on such content.