MongoDB: aggregating fields from arrays of subdocuments - mongodb

Question

I have a mongodb collection called Events, containing baseball games. Here is an example of one record in the table:
{
"name" : "Game# 814",
"dateStart" : ISODate("2012-09-28T14:47:53.695Z"),
"_id" : ObjectId("53a1b24de3f25f4443d9747e"),
"stats" : [
{
"team" : ObjectId("53a11a43a8de6dd8375c940b"),
"teamName" : "Reds",
"_id" : ObjectId("53a1b24de3f25f4443d97480"),
"score" : 17
},
{
"team" : ObjectId("53a11a43a8de6dd8375c938d"),
"teamName" : "Yankees",
"_id" : ObjectId("53a1b24de3f25f4443d9747f"),
"score" : 12
}
]
"__v" : 0
}
I need help writing the query that returns standings for all teams. The result set should look like:
{
"team" : ObjectId("53a11a43a8de6dd8375c938d"),
"teamName" : "Yankees",
"wins" : <<number of Yankees wins>>
"losses" : <<number of Yankees losses>>
"draws" : <<number of Yankees draws>>
}
{
"team" : ObjectId("53a11a43a8de6dd8375c940b"),
"teamName" : "Reds",
"wins" : <<number of Reds wins>>
"losses" : <<number of Reds losses>>
"draws" : <<number of Reds draws>>
}
...
Here's the query I've started with...
db.events.aggregate(
{"$unwind": "$stats" },
{ $group : {
_id : "$stats.team",
gamesPlayed : { $sum : 1},
totalScore : { $sum : "$stats.score" }
}}
);
... which returns results:
{
"result" : [
{
"_id" : ObjectId("53a11a43a8de6dd8375c93cb"),
"gamesPlayed" : 125, // not a requirement... just trying to get $sum working
"totalScore" : 1213 // ...same here
},
{
"_id" : ObjectId("53a11a44a8de6dd8375c955f"),
"gamesPlayed" : 128,
"totalScore" : 1276
},
{
"_id" : ObjectId("53a11a44a8de6dd8375c9661"),
"gamesPlayed" : 152,
"totalScore" : 1509
},
....

Answer 1

It would seem advisable for you to keep your "wins", "losses", "draws" within your documents as you create or update them. But it is possible to do with aggregate if a little long winded
db.events.aggregate([
// Unwind the "stats" array
{ "$unwind": "$stats" },
// Combine the document with new fields
{ "$group": {
"_id": "$_id",
"firstTeam": { "$first": "$stats.team" },
"firstTeamName": { "$first": "$stats.teamName" },
"firstScore": { "$first": "$stats.score" },
"lastTeam": { "$last": "$stats.team" },
"lastTeamName": { "$last": "$stats.teamName" },
"lastScore": { "$last": "$stats.score" },
"minScore": { "$min": "$stats.score" },
"maxScore": { "$max": "$stats.score" }
}},
// Calculate by comparing scores
{ "$project": {
"firstTeam": 1,
"firstTeamName": 1,
"firstScore": 1,
"lastTeam": 1,
"lastTeamName": 1,
"lastScore": 1,
"firstWins": {
"$cond": [
{ "$gt": [ "$firstScore", "$lastScore" ] },
1,
0
]
},
"firstLosses": {
"$cond": [
{ "$lt": [ "$firstScore", "$lastScore" ] },
1,
0
]
},
"firstDraws": {
"$cond": [
{ "$eq": [ "$firstScore", "$lastScore" ] },
1,
0
]
},
"lastWins": {
"$cond": [
{ "$gt": [ "$lastScore", "$firstScore" ] },
1,
0
]
},
"lastLosses": {
"$cond": [
{ "$lt": [ "$lastScore", "$firstScore" ] },
1,
0
]
},
"lastDraws": {
"$cond": [
{ "$eq": [ "$lastScore", "$firstScore" ] },
1,
0
]
},
"type": { "$literal": [ true, false ] }
}},
// Unwind the "type"
{ "$unwind": "$type" },
// Group teams conditionally on "type"
{ "$group": {
"_id": {
"team": {
"$cond": [
"$type",
"$firstTeam",
"$lastTeam"
]
},
"teamName": {
"$cond": [
"$type",
"$firstTeamName",
"$lastTeamName"
]
}
},
"owins": {
"$sum": {
"$cond": [
"$type",
"$firstWins",
"$lastWins"
]
}
},
"olosses": {
"$sum": {
"$cond": [
"$type",
"$firstLosses",
"$lastLosses"
]
}
},
"odraws": {
"$sum": {
"$cond": [
"$type",
"$firstDraws",
"$lastDraws"
]
}
}
}},
// Project your final form
{ "$project": {
"_id": 0,
"team": "$_id.team",
"teamName": "$_id.teamName",
"wins": "$owins",
"losses": "$olosses",
"draws": "$odraws"
}}
])
The first part is to "re-shape" the document by unwinding the array and then grouping with "first" and "last" for defining fields for your two teams.
Then you want to $project through those documents and calculate your "wins", "losses" and "draws" for each team in the pairing. The additional thing is adding an array field for the two values true/false is convenient here. If you are on a pre 2.6 version of mongodb the $literal can be replaced with $const which is not documented but does the same thing.
Once you $unwind that "type" array, the documents can be split apart in the $group stage by evaluating whether to choose the "first" or "last" team field values via the use of $cond. This is a ternary operator that evaluates a true/false condition and returns the appropriate value according to that condition.
With a final $project your documents are formed exactly how you want.