Grouping mongoDB

Question

I have a series of documents in MongoDB that look like this:

{ "playerId" : ObjectId("5c58363b5c226c24b0b37860"), "gameId" : ObjectId("5c59697f57ef0f512c1cb228"), "state" : 4 } { "playerId" : ObjectId("5beab425c0d75e5afabc1638"), "gameId" : ObjectId("5c59697f57ef0f512c1cb228"), "state" : 4 }

I would like to aggregate and have the below result:

{ "_id" : ObjectId("5beab425c0d75e5afabc1638"), // the playerId "state" : 4, "opponents": ["","","",""] // all the opponents gameId }

Group them by the playerId, find all games, user is playing and get all the opponents playerId.

Any thoughts?

mickl · Accepted Answer · 2019-02-07 18:56:52Z

You can try below aggregation:

db.col.aggregate([ { $group: { _id: "$playerId", games: { $push: "$gameId" } } }, { $lookup: { from: "col", localField: "games", foreignField: "gameId", as: "games" } }, { $project: { _id: 1, opponents: { $map: { input: { $filter: { input: "$games", cond: { $ne: [ "$$this.playerId", "$_id" ] } } }, in: "$$this.playerId" } } } } ])

Basically you need to start with $group which will give you all the games for every player. Then you can use $lookup to merge those games with initial collection. In the next step you can use $filter to get only documents with opponents (exclude the ones that have the same _id as current player) and $map to get only playerIds

Prints:

{ "_id" : ObjectId("5beab425c0d75e5afabc1638"), "opponents" : [ ObjectId("5c58363b5c226c24b0b37860") ] } { "_id" : ObjectId("5c58363b5c226c24b0b37860"), "opponents" : [ ObjectId("5beab425c0d75e5afabc1638") ] }

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