MongoDB - calculating average of nested array of objects' attributes

Question

I have a database of teams with nested players as below:

{
 team_id: \"eng1\",
 date_founded: new Date(\"Oct 04, 1896\"),
 league: \"Premier League\",
 point         


        

Answer 1

Since the players field is an array, trying to access its member with $players.age is too confusing, mongo doesn't know which element of the array you want to access. Then come $unwind to the rescue, it will make each element of the array to become the embedded element of the field players. If you do $unwind with the document "Manchester United", you will have something like this

{ "_id" : ObjectId("5666fbbd755e59eab7a3e05e"), "team_id" : "eng1", "date_founded" : ISODate("1896-10-03T17:00:00Z"), "league" : "Premier League", "points" : 62, "name" : "Manchester United", "players" : { "p_id" : "Rooney", "goal" : 85, "caps" : 125, "age" : 28 } }
{ "_id" : ObjectId("5666fbbd755e59eab7a3e05e"), "team_id" : "eng1", "date_founded" : ISODate("1896-10-03T17:00:00Z"), "league" : "Premier League", "points" : 62, "name" : "Manchester United", "players" : { "p_id" : "Scholes", "goal" : 15, "caps" : 225, "age" : 28 } }
{ "_id" : ObjectId("5666fbbd755e59eab7a3e05e"), "team_id" : "eng1", "date_founded" : ISODate("1896-10-03T17:00:00Z"), "league" : "Premier League", "points" : 62, "name" : "Manchester United", "players" : { "p_id" : "Giggs", "goal" : 45, "caps" : 359, "age" : 38 } }`

The field players of the above document have 3 element, so you will have 3 documents with same property like the original document, but the element in the array have moved up to be come embedded document of players. Then access to the age of a player is easy with $players.age because its an embedded document.

Final query

cursor = db.teams.aggregate([
    { $unwind: "$players" },
    { $group : { _id: "$name", avgAge : {  $avg : "$players.age" } } }
]);