[FIXED] Django admin with Nullable ForeignKey in Model but why required in admin page?

Issue

I am using Django and Python to create a custom admin interface for a model named ContentProgress. The ContentProgress model has a nullable ForeignKey field named custom_user_content.

However, when I try to save or edit a ContentProgress instance in the Django admin without providing a value for custom_user_content, I encounter an error stating "This field is required for Custom user Content."

# Admin class
class ContentProgressAdmin(admin.ModelAdmin, ExportCsvMixin):
    list_display = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'created_at', 'updated_at']
    fields = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'real_seconds_listened', 'custom_user_content']
    search_fields = ['user__username', 'lesson__name', 'user__email']
    list_filter = ['done', ('custom_user_content', RelatedDropdownFilter), ('user__company', RelatedDropdownFilter)]
    autocomplete_fields = ['user', 'lesson', 'custom_user_content']
    actions = ['export_as_csv']

admin.site.register(ContentProgress, ContentProgressAdmin)

# Model
class ContentProgress(Progress):
    lesson = models.ForeignKey(MediaFile, on_delete=models.CASCADE)
    custom_user_content = models.ForeignKey(CustomUserContent, null=True, on_delete=models.CASCADE)

    def __str__(self):
        return f'{self.lesson.name} - {int(self.in_percent * 100)}%'

Despite setting null=True for the custom_user_content field, I still face validation issues in the Django admin.

How can I properly handle nullable ForeignKey fields in the admin interface to avoid this error?

Solution

From the doc

Note that this is different than null. null is purely database-related, whereas blank is validation-related. If a field has blank=True, form validation will allow entry of an empty value. If a field has blank=False, the field will be required.

So, you should set blank=True in your models as well in this case to avoid the validation errors.

class ContentProgress(Progress):
    lesson = models.ForeignKey(MediaFile, on_delete=models.CASCADE)
    custom_user_content = models.ForeignKey(
        CustomUserContent,
        null=True,
        blank=True,
        on_delete=models.CASCADE
    )

    def __str__(self):
        return f'{self.lesson.name} - {int(self.in_percent * 100)}%'

See this SO post to get the in-depth idea of null vs blank

Answered By - JPG