time_booking.rb

class TimeBooking < ActiveRecord::Base
  unloadable

  attr_accessible :started_on, :stopped_at, :time_entry_id, :time_log_id, :virtual, :project
  belongs_to :project
  belongs_to :time_log
  belongs_to :time_entry, :dependent => :delete
  has_one :virtual_comment, :dependent => :delete

  validates_presence_of :time_log_id
  validates :time_entry_id, :presence => true, :unless => Proc.new { |tb| tb.virtual }
  validates_associated :virtual_comment, :if => Proc.new { |tb| tb.virtual }

  # <SOME OTHER METHODS>

  def hours_spent
    ((stopped_at.to_i - started_on.to_i) / 3600.0).to_f
  end
end

time_log.rb

class BookingError < StandardError
  attr_reader :message
  def initialize(message)
    @message = message
  end
end

class TimeLog < ActiveRecord::Base
  unloadable

  attr_accessible :user_id, :started_on, :stopped_at, :project_id, :comments, :issue_id, :spent_time
  attr_accessor :issue_id, :spent_time
  belongs_to :user
  has_many :time_bookings, :dependent => :delete_all
  has_many :time_entries, :through => :time_bookings

  def initialize(arguments = nil, *args)
    super(arguments)
  end

  # <SOME OTHER METHODS>

  # returns the sum of bookable time of an time entry
  # if log was not booked at all, so the whole time is bookable
  def bookable_hours
    time_booked = 0
    time_bookings.each do |tb|
      time_booked += tb.hours_spent
    end
    hours_spent - time_booked
  end
end