Django admin model relationship

Question

So, I am building a Store website, I got 2 apps, one app called Store, it has two models "Store" and "UnitStore". The second app is called Product whereas it got single models called Store.

In Product admin, I am using a formfield_for_foreignkey method to show StoreUnit and Store like this: "Store_name | Unit_name"

So, my problem is that when I try to create a product, it always ask for a UnitStore's foreign key, but not all products comes from a UnitStore, some product can only be found in a Store, therefore, I am not able to create a instance.

- Store > Models.py 

class Store(models.Model):

    store_name = models.CharField(max_length=200)
    created_at = models.DateTimeField(auto_now_add=True)
        updated_at = models.DateTimeField(auto_now=True)

    def __str__(self):
        return self.store_name 

class UnitStore(models.Model):

    store_unit_name = models.CharField(max_length=200)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    store = models.ForeignKey(
        Store,
        related_name='units',
        on_delete=models.CASCADE,
    )

    def __str__(self):
        return self.store_unit_name 


Product > Models.py

class Product(models.Model):

    product= models.CharField(max_length=200)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    store_unit_name = models.ForeignKey(
        StoreUnit,
        related_name='units',
        on_delete=models.CASCADE,
    )

    def __str__(self):
        return self.product

- Product > Admin.py

class CustomProduct(admin.ModelAdmin):
    model = Product
    list_display = ('product', 'created_at', 'aupdated_at',
                    'store_unit_name',)
    list_select_related = ('store_unit_name')
    fieldsets = (
        ("Prodcut's Name:", {
            'fields': ('product',)}),
        ('Store:', {
            'fields': ('store_unit_name',)}),  
    )

    def save_model(self, request, obj, form, change):
        obj.created_by = request.user
        super().save_model(request, obj, form, change)

Should I add another foreign key to the Product models, like the example below. therefore I should be able to access the Product by: product.store.store_name.

class Product(models.Model):

    product= models.CharField(max_length=200)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    store_unit_name = models.ForeignKey(
        StoreUnit,
        related_name='units',
        on_delete=models.CASCADE,
    )
  store= models.ForeignKey(
        Store,
        related_name='stores',
        on_delete=models.CASCADE,
    )

Answer 1

You need to have a Store model that captures both stores and unit stores and its __str__ method should print the main store name always and the unit store name if applicable:

class Store(models.Model):

    store_name = models.CharField(max_length=200)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    main_store = models.ForeignKey(
        'Store',
        null=True,
        related_name='units',
        on_delete=models.CASCADE,
    )

    def __str__(self):
        ret_value = self.main_store.store_name + " | " if self.main_store else ""
        return ret_value + self.store_name

Once you have the model like this you can just have a single foreign key pointing at either a store or a unit store.

class Product(models.Model):

    product = models.CharField(max_length=200)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    store= models.ForeignKey(
        Store,
        related_name='products',
        on_delete=models.CASCADE,
    )