aggregation and group in Mongodb

Question

I am getting below documents after my $project , but now I need to find out the average of the transactions. Below are the documents after my '$project', before '$group'

{ "name" : "AAA", "transactions" : [ { "amount" : 500000 }, { "amount" : 12700000 }, { "amount" : 27500000 } ] } { "name" : "BBBB", "transactions" : [ { "amount" : 2500000 }, { "amount" : 5500000 }, { "amount" : 18000000 } ] } { "name" : "CCCC", "transactions" : [ { "amount" : 10000000 }, { "amount" : 5000000 }, { "amount" : 1000000 } ] }

I tried something like this.

 {$group:{"_id":"$name", average:{$avg:"$transactions.amount"}}}

I am printing name values in _id field but the 'average' is printing 'null'

here is the output I am getting

{ "_id" : "AAA", "average" : null } { "_id" : "BBB", "average" : null } { "_id" : "CCC", "average" : null }

what am I missing?

Roger · Accepted Answer · 2016-07-04 01:56:36Z

The problem with you $group stage on the aggregation framework, is that there is nothing to sum up to make an average. I mean, you are working around the "pure" array of values, but the $avg expression is not intelligent enough to sum up all the content of the array. You are trying to get an average of the array as a whole. The correct way to achieve this is doing an $unwind on a previous stage so you got documents like this

{ "name" : "AAA", "transactions" : {"amount" : 500000} } { "name" : "AAA", "transactions" : {"amount" : 12700000} } { "name" : "AAA", "transactions" : {"amount" : 27500000} }

now, you can do your $group stage that should look like

{$group:{"_id":"$name", average:{$avg:"$transactions.amount"}}}

this will make an average for each element contained in the transactions array that you $unwind before to have a document for each of the member.

I think you are taking a mongodb course on mongo university, good luck!

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