class TestAggregation
  def self.data
    command = {
      aggregate: 'cohort_generics',
      pipeline: [
        {
          '$match' => {
            product_id: 2,
            created_at: {
              '$gte' => 1.month.ago
            }
          }
        },
        {
          '$group' => {
            _id: {
              startDate: '$start_date',
              type: '$_type',
              bucket_type: '$bucket_type'
            },
            people: { '$sum' => 1 }
          }
        }
      ]
    }

    Mongoid.default_session.command(command)['result']
  end
end

Moped::Errors::OperationFailure: The operation: #<Moped::Protocol::Command
  @length=290
  @request_id=2
  @response_to=0
  @op_code=2004
  @flags=[]
  @full_collection_name="usercycle.$cmd"
  @skip=0
  @limit=-1
  @selector={:aggregate=>"cohort_generics", :pipeline=>[{"$match"=>{:product_id=>2, :created_at=>{"$gte"=>Mon, 27 Aug 2012 18:07:53 UTC +00:00}}}, {"$group"=>{:_id=>{:startDate=>"$start_date", :type=>"$_type", :bucket_type=>"$bucket_type"}, :people=>{"$sum"=>1}}}]}
  @fields=nil>
failed with error 16002: "exception: can't create Value of BSON type Symbol"

See https://github.com/mongodb/mongo/blob/master/docs/errors.md
for details about this error.