MongoDB aggregation problems

Question

I'm trying to count average BMI for every nationality in my database. I had to cast weight and height do double cause $avg was returning null

db.people.aggregate
(
    {
        $group:
       {
            _id:"$nationality",
            avgBMi:
            {
                $avg:
                {
                    $divide:
                    [
                        {$toDouble:"$weight",$pow:[{$toDouble:"$height"},2]}
                    ]
                }
            } 
        }
    }   
)

but im getting errors and i dont know how to deal with them

"errmsg" : "An object representing an expression must have exactly one 
field: { $toDouble: \"$weight\", $pow: [ { $toDouble: \"$height\" }, 2.0 ] }"

then I read about match and group so I made

db.people.aggregate
(
   {$group:
   {
       _id:"$nationality"
   }},
   {
       $match:{
           avgBMi:
            {
                $avg:
                {
                    $divide:
                    [
                        {$toDouble:"$weight",$pow:[{$toDouble:"$height"},2]}
                    ]
                }
            }  

       }
   }
)

and i get

"errmsg" : "unknown operator: $avg"

here's a sample of my collection

{
  "_id": {
    "$oid": "5be18d5cedc30c3a396e3651"
  },
  "sex": "Female",
  "first_name": "Frances",
  "last_name": "Romero",
  "job": "Project Manager",
  "email": "fromeroc@ted.com",
   "location": {
     "city": "Yantang",
     "address": {
      "streetname": "Holy Cross",
       "streetnumber": "33801"
     }
   },
   "description": "non velit nec nisi vulputate nonummy maecenas tincidunt 
    lacus at velit",
    "height": "179.89",
    "weight": "67.8",
    "birth_date": "1954-03-25T11:51:38Z",
    "nationality": "China"
}

Can u guys please help me?

Answer 1

You were close on your first attempt, but the weight and height conversions need to be separate objects:

db.people.aggregate([
   {$group:
   {
       _id:"$nationality",
       avgBMi:
        {
            $avg:
            {
                $divide:
                [
                    {$toDouble:"$weight"},
                    {$pow:[{$toDouble:"$height"},2]}
                ]
            }
        }             
   }}
])

Output:

{ "_id" : "China", "avgBMi" : 0.0020951525521518315 }