models.py

class SkillHistory(models.Model):
    """
        The reason why a Skill is acquired or not,
        or not yet, when and by who/how

    """

    skill = models.ForeignKey(Skill)
    """The Skill to validate"""
    student = models.ForeignKey('users.Student')
    """The Student concerned by this Skill"""
    datetime = models.DateTimeField(auto_now_add=True)
    """The date the Skill status was created"""
    value = models.CharField(max_length=255, choices=(
        ('unknown', 'Inconnu'),
        ('acquired', 'Acquise'),
        ('not acquired', 'None Acquise'),
    ))
    """The Skill status : unknown, acquired or not acquired"""

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    reason_object = GenericForeignKey('content_type', 'object_id')
    reason = models.CharField(max_length=255)
    """Why the Skill is validated or not"""

    by_who = models.ForeignKey(User)

    class Meta:
        ordering = ['datetime']