django left join with null
- by SledgehammerPL
The model:
class Product(models.Model):
name = models.CharField(max_length = 128)
def __unicode__(self):
return self.name
class Receipt(models.Model):
name = models.CharField(max_length=128)
components = models.ManyToManyField(Product, through='ReceiptComponent')
class Admin:
pass
def __unicode__(self):
return self.name
class ReceiptComponent(models.Model):
product = models.ForeignKey(Product)
receipt = models.ForeignKey(Receipt)
quantity = models.FloatField(max_length=9)
unit = models.ForeignKey(Unit)
def __unicode__(self):
return unicode(self.quantity!=0 and self.quantity or '') + ' ' + unicode(self.unit) + ' ' + self.product.genitive
The idea:
there are a components on stock. I'd like to find out which recipes I can made with components which I have.
It's not easy - but possible - I made a SQL view, which gets the solution. But I'm learning python and Django so I'd like to make it Django-style ;D
The concept of solution:
get the set of recipes which has at last one component:
list_of_available_components = ReceiptComponent.objects.filter(product__in=list_of_available_products).distinct()
list_of_related_receipts = Receipt.objects.filter(receiptcomponent__in = list_of_available_components).distinct()
get recipes (from list_of_related_receipts) which has not at last one component
list_of_incomplete_recipes = (SELECT * FROM drinkbook_receiptcomponent LEFT JOIN drinkstore_stock_products USING(product_id) WHERE drinkstore_stock_products.stock_id IS NULL AND receipt_id IN (SELECT receipt_id FROM drinkbook_receiptcomponent JOIN drinkstore_stock_products USING(product_id)))
get recipes (from list_of_related_receipts) which are not in "list_of_incomplete_recipes"
