Django Model set foreign key to a field of another Model

Question

Is there any way to set a foreign key in django to a field of another model?

For example, imagine I have a ValidationRule object. And I want the rule to define what field in another model is to be validated (as well as some other information, such as whether it can be null, a data-type, range, etc.)

Is there a way to store this field-level mapping in django?

Answer 1

I haven't tried this, but it seems that since Django 1.0 you can do something like:

class Foo(models.Model):
    foo = models.ForeignKey(Bar, to_field='bar')

Documentation for this is here.

Answer 2

Yes and no. The FK relationship is described at the class level, and mirrors the FK association in the database, so you can't add extra information directly in the FK parameter.

Instead, I'd recommend having a string that holds the field name on the other table:

class ValidationRule(models.Model):
    other = models.ForeignKey(OtherModel)
    other_field = models.CharField(max_length=256)

This way, you can obtain the field with:

v = ValidationRule.objects.get(id=1)
field = getattr(v, v.other_field)

Note that if you're using Many-to-Many fields (rather than a One-to-Many), there's built-in support for creating custom intermediary tables to hold meta data with the through option.

Answer 3

You need to use "to_field" in "models.ForeignKey()" to set other field in other model:

ForeignKey.to_field

The field on the related object that the relation is to. By default, Django uses the primary key of the related object. If you reference a different field, that field must have unique=True.

For example, as a foreign key, "categories" field in "Product" model references "name" field in "Category" model as shown below. *Be careful, the referenced field "name" in "Category" model needs "unique=True" or "primary_key=True" otherwise there is an error:

# "models.py"

from django.db import models
                                            # "unique=True" or
class Category(models.Model):               # "primary_key=True" are needed 
    name = models.CharField(max_length=100, unique=True)
    
class Product(models.Model):
    name = models.CharField(max_length=100)                
    categories = models.ForeignKey(
        Category, 
        to_field='name', # ← Here
        on_delete=models.PROTECT
    )