Storing factory-boy RelatedFactory Object on Parent Factory

Question

I have two Django models (Customer and CustomerAddress) that both contain ForeignKeys to each other. I am using factory-boy to manage creation of these models, and cannot save a child factory instance onto the parent factory (using relationships defined using the RelatedFactory class).

My two models:

class ExampleCustomerAddress(models.Model):
    # Every customer mailing address is assigned to a single Customer,
    # though Customers may have multiple addresses.
    customer = models.ForeignKey('ExampleCustomer', on_delete=models.CASCADE)

class ExampleCustomer(models.Model):
    # Each customer has a single (optional) default billing address:
    default_billto = models.ForeignKey(
        'ExampleCustomerAddress',
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
        related_name='+')

I have two factories, one for each model:

class ExampleCustomerAddressFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = ExampleCustomerAddress

    customer = factory.SubFactory(
        'ExampleCustomerFactory',
        default_billto=None)  # Set to None to prevent recursive address creation.

class ExampleCustomerFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = ExampleCustomer

    default_billto = factory.RelatedFactory(ExampleCustomerAddressFactory,
                                            'customer')

When creating a ExampleCustomerFactory, default_billto is None, even though a ExampleCustomerAddress has been created:

In [14]: ec = ExampleCustomerFactory.build()

In [15]: ec.default_billto is None
Out[15]: True

(When using create(), a new ExampleCustomerAddress exists in the database. I am using build() here to simplify the example).

Creating an ExampleCustomerAddress works as expected, with the Customer being automatically created:

In [22]: eca = ExampleCustomerAddressFactory.build()

In [23]: eca.customer
Out[23]: <ExampleCustomer: ExampleCustomer object>

In [24]: eca.customer.default_billto is None
Out[24]: True  <-- I was expecting this to be set to an `ExampleCustomerAddress!`.

I feel like I am going crazy here, missing something very simple. I get the impression I am encountering this error because of how both models contain ForeignKeys to each other.

Sutlej

Answer 1

First, a simple rule of thumb: when you're following a ForeignKey, always prefer a SubFactory; RelatedFactory is intended to follow a reverse relationship.

Let's take each factory in turn.

ExampleCustomerAddressFactory

When we call this factory without a customer, we'll want to get an address, linked to a customer, and used as the default address for that customer.

However, when we call it with a customer, don't alter it.

The following would work:

class ExampleCustomerAddressFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = ExampleCustomerAddress

    # Fill the Customer unless provided
    customer = factory.SubFactory(
        ExampleCustomerFactory,
        # We can't provide ourself there, since we aren't saved to the database yet.
        default_billto=None,
    )

    @factory.post_generation
    def set_customer_billto(obj, create, *args, **kwargs):
        """Set the default billto of the customer to ourselves if empty"""
        if obj.customer.default_billto is None:
            obj.customer.default_billto = obj
            if create:
                obj.customer.save()

Here, we'll set the newly created customer's value to "us"; note that this logic could also be moved to ExampleCustomerAddress.save().

ExampleCustomerFactory

For this factory, the rules are simpler: when creating a customer, create a default billing address (unless a value has been provided).

class ExampleCustomerFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = ExampleCustomer

    # We can't use a SubFactory here, since that would be evaluated before
    # the Customer has been saved.
    default_billto = factory.RelatedFactory(
        ExampleCustomerAddressFactory,
        'customer',
    )

This factory will run as follows:

1. Create the ExampleCustomer instance with default_billto=None;
2. Call ExampleCustomerAddressFactory(customer=obj) with the newly created customer;
3. That factory will create an ExampleCustomerAddress with that customer;
4. The post-generation hook in that factory will then detect that the customer has no default_billto, and will override it.

Notes

• I didn't test this, so some typos or minor bugs could occur;
• It's up to you to decide which factory is declared first, using the target factory's path instead of a direct reference;
• As stated above, the logic to set the default billing address of a customer when it's empty and an address is added to that customer could be moved to your model's .save() method.

Morena

Thank you sooo much! This worked perfectly. For some reason, I got hung up adding the post-generation hook onto the CustomerFactory. It never occurred to me to place it onto the CustomerAddressFactory. One quick edit to your answer: the @factory.post_declaration decorator should be @factory.post_generation. – Sutlej