new_contents: """
  Implements the per-domain data retention policy.
  
  The goal is to have a single place where the policy is defined.  This is
  complicated by needing to apply this policy both to the database and to log
  files.  Additionally, we want to use an efficient query for the database,
  rather than iterating through messages one by one.
  
  The code in this module does not actually remove anything; it just identifies
  which items should be kept or removed.
  """
  
  import sys
  import operator
  
  from django.utils     import timezone
  from django.db.models import Q
  from datetime         import datetime, timedelta
  from zephyr.models    import Realm, UserMessage
  
  # Each domain has a maximum age for retained messages.
  #
  # FIXME: Move this into the database.
  max_age = {
      'customer1.invalid': timedelta(days=31),
  }
  
  def should_expunge_from_log(msg, now):
      """Should a particular log entry be expunged?
  
         msg: a log entry dict
         now: current time for purposes of determining log entry age"""
  
      # This function will be called many times, but we want to compare all
      # entries against a consistent "current time".  So the caller passes
      # that time as a parameter.
  
      # FIXME: Yet another place where we compute the domain manually.
      # See #260.
      user = msg.get('sender_email')
      if user is None:
          user = msg.get('user')
      if user is None:
          # Avoid printing the entire message, but give enough information to find it later.
          print >>sys.stderr, "WARNING: Can't get user for message at", msg['timestamp']
          return False
      domain = user.split('@', 1)[1]
  
      if domain not in max_age:
          # Keep forever.
          return False
  
      age = now - datetime.fromtimestamp(msg['timestamp'])
      return age > max_age[domain]
  
  def get_UserMessages_to_expunge():
      """Fetch all UserMessages which should be expunged from the database.
  
         After deleting these, you may also want to call
         Message.remove_unreachable()."""
      # Unlike retain_in_log, this handles all messages at once, so we
      # use the actual current time.
      now = timezone.now()
      queries = [Q(user_profile__realm   = realm,
                   message__pub_date__lt = now - max_age[realm.domain])
                 for realm in Realm.objects.all()
                 if  realm.domain in max_age]
  
      if not queries:
          return UserMessage.objects.none()
  
      # Return all objects matching any of the queries in 'queries'.
      return UserMessage.objects.filter(reduce(operator.or_, queries))
  
subject: Implement a framework for data retention policies