Hi!
I'm having trouble overriding some prefetched data, I hope someone can help
me here. I'll try to explain my issue with the examples provided from the
Docs (
https://docs.djangoproject.com/en/dev/ref/models/querysets/#prefetch-related).
The base models from the examples below were taken from there.
So... Let's suppose I'm running a Pizzeria, and I have the following models:
class Topping(models.Model):
name = models.CharField(max_length=30)
class Pizza(models.Model):
name = models.CharField(max_length=50)
toppings = models.ManyToManyField(Topping)
My goal is to fetch my Pizzas, serialize them, and store it into an
ElasticSearch cluster.
If I want to fetch all my Pizzas from DB, I run the following:
pizzas = Pizza.objects.all()
But also with the Pizza object, I want to (pre)fetch all toppings. In that
case, I should run:
Pizza.objects.all().prefetch_related('toppings')
This works great, but I might want to add some filters to the toppings
fetched for any reason (They are out of stock, for example). I can do this
by adding the field in_stock to my model Topping:
class Topping(models.Model):
name = models.CharField(max_length=30)
in_stock = models.BooleanField()
With this Topping model, I can use the Prefetch object:
pizzas = Pizza.objects.all().prefetch_related(
Prefetch('toppings', queryset=Topping.objects.filter
(in_stock=True)
)
Until now everything works as expected (even the serialization). But my
problem begins when I try to do the same stock control with the Pizza
object. Just like I did with Topping, I will also add the in_stock field to
the Pizza Model. The new Pizza Model then is:
class Pizza(models.Model):
name = models.CharField(max_length=50)
toppings = models.ManyToManyField(Topping)
in_stock = models.BooleanField()
If my Pizza is out of stock, I don't need to know which toppings are
available and which are not. After the query execution I expected a list of
all Pizzas and, if the pizza is available, the list of Toppings available
for each pizza. I wasn't able to make this behavior using the Prefetch
object. As a workaround, I tried the following:
pizzas = Pizza.objects.all().prefetch_related(
Prefetch('toppings', queryset=Topping.objects.filter
(in_stock=True)
)
for pizza in pizzas:
if not pizza.in_stock:
pizza.toppings = []
Running the following, Python/Django raised an error saying that I should
use this instead:
pizza.toppings.set([])
But even when I the code above (that runs without raising an error), every
time I serialize my pizza object, the full toppings list gets serialized
instead of the empty array previously setted.
I am using an HyperlinkedModelSerializer to serialize my data.
Can anyone help me? How can I serialize the empty array instead of the list
retrieved from the db?
Thanks in advance
Cesar
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