from django.db import models
from django.db.models import F, Q


class FuelPurchase(models.Model):
    purchase_date = models.DateField(null=False, blank=False, verbose_name="Purchase Date")
    total_cost = models.DecimalField(null=False, blank=False, max_digits=10, decimal_places=2, verbose_name="Total Cost (€)")
    price_per_litre = models.DecimalField(null=False, blank=False, max_digits=6, decimal_places=3, verbose_name="Price per Litre (€)")
    amount_litres = models.DecimalField(null=False, blank=False, max_digits=7, decimal_places=2, verbose_name="Amount (litres)")

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=[
                    'purchase_date',
                    'total_cost',
                    'price_per_litre',
                    'amount_litres',
                ],
                name='unique_fuel_purchase'
            ),
            models.CheckConstraint(
                check=Q(total_cost__gt=0) & Q(total_cost__lt=100),
                name="total_cost_range"
            ),
            models.CheckConstraint(
                check=Q(price_per_litre__gt=0) & Q(price_per_litre__lt=5),
                name="price_per_litre_range"
            ),
            models.CheckConstraint(
                check=Q(amount_litres__gt=0) & Q(amount_litres__lt=100),
                name="amount_litres_range"
            ),
        ]
        verbose_name = "Fuel Purchase"
        verbose_name_plural = "Fuel Purchases"

    def save(self, *args, **kwargs):
        if not self.amount_litres and self.price_per_litre:
            self.amount_litres = self.total_cost / self.price_per_litre
        super().save(*args, **kwargs)

    def __str__(self):
        date_str = self.purchase_date.strftime("%d.%m.%Y")
        return f"{date_str} : {self.total_cost} € - {self.price_per_litre} €/L"