--- slug: "django-multiple-database-foreignkey" title: "Creating a ForeignKey Across Databases in Django 2.0" description: "※ This does not mean creating constraints in an RDB. It simply means creating a ForeignKey field that works for now." url: "https://www.ytyng.com/en/blog/django-multiple-database-foreignkey" publish_date: "2018-05-15T03:24:56Z" created: "2018-05-15T03:24:56Z" updated: "2026-02-27T05:38:55.349Z" categories: ["Django"] keywords: "" featured_image_url: "https://media.ytyng.com/resize/20230812/4294209815914e9a99562ba3d51bfdc5.png.webp?width=768" has_video: false has_music: false video_urls: [] music_urls: [] lang: "en" --- # Creating a ForeignKey Across Databases in Django 2.0

※ This does not mean creating constraints in an RDB. It simply means creating a ForeignKey field that works for now.

In Django 2.0, when you try to create a ForeignKey to a model that uses a different database, you get the following error when saving:

Cannot assign "<YourModel>": the current database router prevents this relation.

This error prevents saving.

In such cases, you can return True in the allow_relation method of the database router:

def allow_relation(self, obj1, obj2):
    if {obj1._meta.app_label, obj2._meta.app_label} \
            == {'my_app_1', 'my_app_2'}:
        return True
↑ This might work if you set everything to True, but if you want to judge by app_label, it would look something like this.
The default is None

https://docs.djangoproject.com/en/2.0/topics/db/multi-db/#allow_relation

For the model that uses the ForeignKey:

related_model = models.ForeignKey(
    YourModel, on_delete=models.DO_NOTHING,
    db_constraint=False, null=True, blank=True)

If you set db_constraint=False like this (otherwise, you cannot migrate),

it will work as a ForeignKey.

However, as indicated in the code, there will be no constraints on the DB.