Create MongoDB fields with names based on sub-document values using aggregation pipeline? -

given mongodb collection following document structure:

{     array_of_subdocs:          [              {                  animal: "cat",                  count: 10              },              {                  animal: "dog",                  count: 20              },               ...          ] } 

where each document contains array of sub-documents, want transform collection documents of structure:

{     cat: { count: 10 },     dog: { count: 20 },     ... } 

where each sub-document value of new field in main document named after 1 of values within sub-document (in example, values of animal field used create name of new fields, i.e. cat , dog).

i know how eval javascript snippet. it's slow. question is: how can done using aggregation pipeline?

according this feature request , solution, new feature added functionality - function called arraytoobject.

db.foo.aggregate([   {$project: {     array_of_subdocs: {       $arraytoobject: {         $map: {           input: "$array_of_subdocs",           as: "pair",           in: ["$$pair.animal", "$$pair.count"]         }       }     }   }} ]) 

but @ moment, no solution. suggest change data structure. far see there many feature requests labeled major not done years.