MongoDB aggregation result as a nested Json

This is a piece of my MongoDB collection : { “_id” : ObjectId(“55b0ba203a20b54e3b1e09e4”), “i” : 0, “x” : 1, “id” : 2, “info” : { “j” : 0 } } { “_id” : ObjectId(“55b0ba203a20b54e3b1e09e5”), “i” : 1, “…