Return array index from $elemMatch query

Question

Say I have a document like this

{
  title : 'myTitle',
  favorites : [{name : 'text', number : 6}, {name : 'other', number : 4}]
}

I would like to return the array index (if any) from where the embedded document was retrieved within the favorites array.

Say I have the following query

 {title : 'myTitle'}

and the projection

 {favorites : {$elemMatch : {name : 'text', number : 6} }}

if the projection returns the document AND it contains a favorites array with the sub document, is there a way to know at which index that sub document was found? Which in this case is index 0.

The reason I would like the index is because as soon as I retrieve the document, I proceed to update it, and it would boost the performance if I had a specific index to update instead of using $elemMatch again which would cause mongo to iterate through all the array entries until finding the document that matches.

Answer 1

Unfortunately, it is not possible using the $elemMatch operator. If you are using mongodb 3.2, you could make use of the $unwind operator and aggregate instead of doing a find()

db.collection.aggregate([
{$match:{"favorites.name":"text","favorites.number":6}},
{$unwind:{"path":"$favorites","includeArrayIndex":"index"}},
{$match:{"favorites.name":"text","favorites.number":6}}
])

The document would be returned, with its array index in the field - index. If you want the entire document, with other array elements, you would have to $group, after $unwind instead of $match.

For previous versions, iterating the array in the client side, and getting the sub document's index would be the way to go.