Mongodian aggregation frame project with conditions

Question

I have a collection of log entries that look like this:

{ _id: 5141aff1a1d24c991a000002, date: 2012-02-23 00:00:00 UTC, details: "sdfd", value: 250, clinic_id: "513e2227a1d24ceab3000001" } 

I want to receive a report on monthly loan and debit amounts. Like this:

 [ {"credit"=> -229, "debit" => 0 "month"=>1}, {"credit"=> -229, "debit" => 0 "month"=>2}, {"credit"=> -229, "debit" => 0 "month"=>3}, {"credit"=> -229, "debit" => 0 "month"=>4}, {"credit"=> -229, "debit" => 0 "month"=>5}, {"credit"=> -229, "debit" => 0 "month"=>6}, {"credit"=> -229, "debit" => 0 "month"=>7}, {"credit"=> 0, "debit" => 300 "month"=>8}, {"credit"=> 0, "debit" => 300 "month"=>9}, {"credit"=> 0, "debit" => 300 "month"=>10}, {"credit"=> 0, "debit" => 300 "month"=>11}, {"credit"=> 0, "debit" => 300 "month"=>12} 

]

To do this, I plan to use an aggregation structure.

• How to assign $ value credit when $ value <= 0?
• How to assign $ value to debit when $ value> = 0?
• How do I group this?

I have it:

 BookKeepingEntry.collection.aggregate( [ { "$match" => { "clinic_id" => self.clinic.id } }, { "$project" => { "credit" => { what here? }, "debit" => { What here?} "month" => { "$month" => "$date" } } }, { "$group" => {} } { "$sort" => { "month" => 1 } } ] ) 

Answer 1

 BookKeepingEntry.collection.aggregate( { $project: { _id: 0, credit : { $cond: [ {$lt: ['$value',0]}, '$value', 0 ] }, debit : { $cond: [ {$gt: ['$value',0]}, '$value', 0 ] }, month : { $month : "$date" } }}, { $group : {_id: {Month: "$month"} , CreditSum: {$sum: "$credit"}, DebitSum: {$sum: "$debit"}} }, { $sort : { "_id.Month" : 1 } } );