Re: Overriding Prefetched objects in Serialization

Hi Barddes,

Bc of you alter model, so let try to command migration, makemigration.

On Fri, Dec 27, 2019, 00:46 Cesar Bueno <barddes@gmail.com> wrote:

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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