class CommonAbstractModel(models.Model):
    """
    Common ABC for most models.
    Provides created/updated_at, created/updated_by, and active/inactive status.
    """
    created_at = models.DateTimeField(
        default=datetime.datetime.now(), editable=False)
    updated_at = AutoDateTimeField(editable=False)
    created_by = models.ForeignKey(User, blank=True, editable=False,
        related_name="creates")
    updated_by = models.ForeignKey(User, blank=True, editable=False,
        related_name="updates")
    active = models.BooleanField(default=True)

class LibraryAbstractModel(models.Model):
  """ generic class for library items """
  title = models.CharField(max_length=255)
  slug = models.SlugField()
  description = models.TextField()
  download = models.BooleanField(default=False)
  share = models.BooleanField(default=False)
  email = models.BooleanField(default=False)
  embed = models.BooleanField(default=False)
  library_set = models.ForeignKey('LibrarySet',
      blank=True, related_name='members')
  keywords = TagField()
  publisher = models.ForeignKey('Publisher',
      blank=True, related_name='publications')
  authors = models.ManyToManyField('Author',
      blank=True, related_name='publications')

class Audio(CommonAbstractModel, LibraryAbstractModel):
  """ Audio items in the library """
  runtime = models.CharField(max_length=255)
  transcript = models.TextField()

class LibrarySet(CommonAbstractModel):
  """ Set of materials in the library """
  title = models.CharField(max_length=255)
  slug = models.SlugField()