MongoDB aggregation - project field values as fields

Question

Having some trouble getting my data into a tricky format.

Documents have these fields: subject, importance [high,medium,low]

sample document:

{_id: "", subject: "red", importance: "high"}

I like to return data that looks like:

[{_id: subject, high: 5, medium: 6, low: 3}] 

The numbers correspond with the number of documents with each importance level

This is what I have so far:

{
$group: {
  _id: {subject: "$subject", importance: "$importance"},
  count: {$sum: 1},
  }
 },
 {
   $group: {
     _id: "$_id.subject",
     data: {
       $push: {
          importance: "$_id.importance", count: "$count"
       }
     }
   }
 }

Is there a way that I can use the value of $_id.importance and make that the key and have $count as the value?

Answer 1

You could aggregate as below:

• $group by subject and importance, get the respective counts.
• Then comes the tricky part, the conditional $project, it would grow linearly with respect to the number of options the importance field could hold. currently it is three - high, low and medium.
• $group the result back again by subject and use the $sum operator to accumulate the counts for the different values of the importance field.

sample code:

db.t.aggregate([
{$group:{"_id":{"subject":"$subject",
                "importance":"$importance"},
         "count":{$sum:1}}},
{$project:{"_id":0,
           "subject":"$_id.subject",
           "result":{$cond:[
                           {$eq:["$_id.importance","high"]},
                           {"high":"$count"},
                           {$cond:[{$eq:["$_id.importance","low"]},
                                   {"low":"$count"},
                                   {"medium":"$count"}]}]}}},
{$group:{"_id":"$subject",
         "low":{$sum:"$result.low"},
         "medium":{$sum:"$result.medium"},
         "high":{$sum:"$result.high"}}},
])

test data:

db.t.insert([
{"subject":"history","importance":"high"},
{"subject":"geography","importance":"low"},
{"subject":"history","importance":"low"},
{"subject":"history","importance":"medium"},
{"subject":"geography","importance":"low"},
{"subject":"history","importance":"low"}
])

result:

{ "_id" : "geography", "low" : 2, "medium" : 0, "high" : 0 }
{ "_id" : "history", "low" : 2, "medium" : 1, "high" : 1 }