module Mongoid::Contextual::Aggregable::Mongo

def aggregates(field)
  result = collection.find.aggregate(pipeline(field), session: _session).to_a
  if result.empty?
    if Mongoid.broken_aggregables
      { "count" => 0, "sum" => nil, "avg" => nil, "min" => nil, "max" => nil }
    else
      Aggregable::EMPTY_RESULT.dup
    end
  else
    result.first
  end
end

def avg(field)
  aggregates(field)["avg"]
end

def max(field = nil)
  block_given? ? super() : aggregates(field)["max"]
end

def min(field = nil)
  block_given? ? super() : aggregates(field)["min"]
end

def pipeline(field)
  db_field = "$#{database_field_name(field)}"
  sort, skip, limit = criteria.options.values_at(:sort, :skip, :limit)
  pipeline = []
  pipeline << { "$match" =>  criteria.exists(field => true).selector }
  pipeline << { "$sort" => sort } if sort && (skip || limit)
  pipeline << { "$skip" => skip } if skip
  pipeline << { "$limit" => limit } if limit
  pipeline << {
    "$group"  => {
      "_id"   => field.to_s,
      "count" => { "$sum" => 1 },
      "max"   => { "$max" => db_field },
      "min"   => { "$min" => db_field },
      "sum"   => { "$sum" => db_field },
      "avg"   => { "$avg" => db_field }
    }
  }
end

def sum(field = nil)
  block_given? ? super() : aggregates(field)["sum"] || 0
end