device_customers_report

devices=Customer.collection.aggregate([
  {
    "$group" => {
      _id: "$device_id",
      count:{"$sum"=>1}
    }
  },
  {
    "$match"=>{
      count:{"$gte"=>3}
    }
  },
  {
    "$sort"=>{
      count:-1
    }
  }     
])

report=[]
devices.each do |d|
  Customer.where(device_id: d["_id"]).each do |c|
    report<<"#{c.name}, #{c.email}"
  end
end