[{"key":1,"completed":true},{"key":2,"completed":false},{"key":3,"completed":true}]

db.collection.aggregate([{"$group":{"_id":null,"completed":{"$sum":{"$cond":[{"$eq":["$completed",true]},1,0]}},"incomplete":{"$sum":{"$cond":[{"$eq":["$completed",false]},1,0]}},"totalCount":{"$sum":1}},}])