from django.db import models

class Category(models.Model):
    name = models.CharField(max_length=255, unique=True)

    def __str__(self):
        return self.name

class Gauge(models.Model):
    serial_number = models.CharField(max_length=40)
    category = models.ForeignKey(Category)

    def __str__(self):
        return '{} ({})'.format(self.category, self.serial_number)
    
    class Meta:
        ordering = ['category']

class Mesure(models.Model):
    moment = models.DateTimeField()
    value = models.DecimalField(max_digits=10, decimal_places=4)
    gauge = models.ForeignKey(Gauge)

    @property
    def category(self):
        return self.gauge.category