How to create a relations between primary key and foreign key in Django models and POST values into DB

I have a Django app which is basically an Online shopping. Right now I have two models: User_details and Extend_user_deatils.

In User_details I have(Username, Firstname, lastname, email, etc..). Now after that i need to extend User_details Model with Other Fields so I have Created another Model Extend_user_deatils consisting of (Mobile , Age) columns.

Now i need to Link those Two Models, for that I have written an `logic with Foreign Key Reference

`
class User_details(models.Model): #Table 1
id = models.AutoField(primary_key=True, db_column='user_id')
username = models.CharField(max_length=50)
first_name = models.CharField(max_length=50, blank=True, null=True)
last_name = models.CharField(max_length=50, blank=True, null=True)
email = models.CharField(max_length=50)

class Extend_user_details(models.Model): #Table 2
user =models.ForeignKey(User_details, on_delete=models.CASCADE,db_column='user_id')
mobile = models.BigIntegerField(blank=True, null=True)
age = models.IntegerField(blank=True, null=True)
`

When I Open Django admin Page and Post the Data it is Working Fine and Values are posting into DB.

When I register as a User from register page . ForeignKey Column(user) from Table2 is showing Null .apart from that all the inputs is posting into DB as shown

Is there any way to solve this issue

Thanks in Advance