MongoDB nested group?

Question

I\'m trying to implement a nested group query in mongodb and I\'m getting stuck trying to add the outer group by. Given the below (simplified) data document:

Answer 1

You will need two groups in this case. The first group generates a stream of documents with one document per term and category:

 { $group : { 
      _id :  { 
        category: "$category",
        term: "$term",
      },
      total: { $sum : 1 } 
   }
 }

A second group will then merge all documents with the same term into one, using the $push operator to merge the categories into an array:

 { $group : { 
      _id :  "$_id.category",
      terms: { 
          $push: { 
              term:"$_id.term",
              total:"$total"
          }
      }
   }
 }