gistfile1.rb

def total_by(obj, date)
  if obj.is_a? String
    group_by = obj
    filter_by = false
  else
    filter_by = obj.class.model_name
    group_by = opposite_association(name).to_s
  end

  query = scoped.where(date: date.beginning_of_month..date.end_of_month)
  query = query.where(filter_by.foreign_key => obj.id) if filter_by
  
  result = query.sum(:time, group: group_by.foreign_key)

  list = group_by.pluralize.to_sym
  if obj.respond_to? list
    obj.send(list).each do |item|
      result[item.id] = 0 unless result.key? item.id
    end
  end

  result
end