Is there a way to query MongoDB collection with an embedded array to return the element's offset within the array

Question

Given the following documents in the school collection:

{
    _id: 1,
    zipcode: "63109",
    students: [
        { name: "john", school: 102, age: 10 },
        { name: "jess", school: 102, age: 11 },
        { name: "jeff", school: 108, age: 15 }
    ]
}
{
    _id: 2,
    zipcode: "63110",
    students: [
        { name: "ajax", school: 100, age: 7 },
        { name: "achilles", school: 100, age: 8 },
    ]
}
{
    _id: 3,
    zipcode: "63109",
    students: [
        { name: "ajax", school: 100, age: 7 },
        { name: "achilles", school: 100, age: 8 },
    ]
}
{
    _id: 4,
    zipcode: "63109",
    students: [
        { name: "barney", school: 102, age: 7 },
        { name: "ruth", school: 102, age: 16 },
    ]
}

Here is the example query in MongoDB docs:

db.schools.find( { zipcode: "63109" },
             { students: { $elemMatch: { school: 108 } } } )

It will return a result like this:

{ "_id" : 1, "students" : [ { "name" : "jeff", "school" : 108, "age" : 15 } ] }

My question is how to get a result like this:

{ "_id" : 1, "students" : [ { "name" : "jeff", "school" : 108, "age" : 15, "offset" : 2 } ] }

As you can see, this result adds a field offset. Has anyone tried this before?

Answer 1

Below is a work around for your problem :

 db.schools.find( { zipcode: "63109" },
    { _id : 1 , students : 1 }).forEach( function(doc){
   var arr = doc.students;
   for( var index = 0 ; index < arr.length ; index++ )
     {
        if( arr[index].school == 108 )
         {
        var d = arr[index];
        d["offset"] = index;
         }
         else
         {
        arr.splice(index, 1);

         }
      }
    });