class Session(models.Model):

    corporal_structures = models.ForeignKey(SingleBodySegment)
    movement = ChainedManyToManyField(
        Movements,
        chained_field = 'corporal_structures',
        chained_model_field = 'corporal_segments'
    )

    metrics = models.ManyToManyField(Metrics, blank=True)

    date_session_begin = models.DateTimeField(default=timezone.now())
    date_session_end = models.DateTimeField(default=timezone.now())
    observations = models.TextField(blank=False)

class Metrics(models.Model):
     name = models.CharField(max_length=255, blank=True)
     equation = models.CharField(max_length=255, blank=True)
     min_value = models.DecimalField(max_digits=5,decimal_places=3)
     max_value = models.DecimalField(max_digits=5,decimal_places=3)

     def __str__(self):
        return "%s" % self.name