Question

I have a model (lets call it Entity) that has an attribute (Attribute) that changes over time, but I want to keep a history of how that attribute changes in the database. I need to be able to filter my Entities by the current value of Attribute in its manager. But because Django (as far as I can tell) won’t let me do this in one query natively, I have created a database view that produces the latest value of Attribute for every Entity. So my model structure looks something like this:

class Entity(models.Model):
    def set_attribute(self, value):
        self.attribute_history.create(value=value)

    def is_attribute_positive(self, value):
        return self.attribute.value > 0

class AttributeEntry(models.Model):
    entity = models.ForeignKey(Entity, related_name='attribute_history')
    value = models.IntegerField()
    time = models.DateTimeField(auto_now_add=True)

class AttributeView(models.Model)
    id = models.IntegerField(primary_key=True, db_column='id', 
        on_delete=models.DO_NOTHING)
    entity = models.OneToOneField(Entity, related_name='attribute')
    value = models.IntegerField()
    time = models.DateTimeField()

    class Meta:
        managed = False

My database has the view that produces the current attribute, created with SQL like this:

CREATE VIEW myapp_attributeview AS
SELECT h1.*
FROM myapp_attributehistory h1
LEFT OUTER JOIN myapp_attributehistory h2
    ON h1.entity_id = h2.entity_id
        AND (h1.time < h2.time
        OR h1.time = h2.time
        AND h1.id < h2.id)
WHERE h2.id IS NULL;

My problem is that if I set the attribute on a model object using set_attribute() checking it with is_attribute_positive() doesn’t always work, because Django may be caching that the related AttributeView object. How I can I make Django update its model, at the very least by requerying the view? Can I mark the attribute property as dirty somehow?

PS: the whole reason I’m doing this is so I can do things like Entity.objects.filter(attribute__value__exact=...).filter(...), so if someone knows an easier way to get that functionality, such an answer will be accepted, too!

Answer 1

I never directly solved the problem, but I was able to sidestep it by changing is_attribute_positiive() to directly query the database table, instead of the view.

def is_attribute_positive(self, value):
    return self.attribute_history.latest().value > 0

So while the view gives me the flexibility of being able to filter queries on Entity, it seems the best thing to do once the object is received is to operate directly on the table-backed model.