Advise on orm modelling in django

I am building a webapp in which users place orders with nested information. I am after some advice on best practice so I can do some more research.

The idea is that a user either loads an existing order, or creates a new order. They then fill in the subsequent order fields, which can be on a one to many basis. My initial thought was to use a variable for order_id and use a foreign key to link the instances of the other classes. However, this seems overcomplicated. order_id would have to be unique and sequential. Is this the best approach to this issue?

class Order(models.Model):
    order_id= xxxx
    created_by = xxxx

class A(models.Model):
    order_id = models.ForeignKey(xxxxx)
    someclassafield = xxxx

class B(models.Model):
    order_id = models.ForeignKey(xxxxx)
    someclassbfield = xxxx

This would give the user the ability to create orders such as below:

Order 1
(Class_A, order_id, someclassafield)
(Class_A, order_id, someclassafield)
(Class_A, order_id, someclassafield)
(Class_B, order_id, someclassbfield)

Order 2
(Class_A, order_id, someclassafield)
(Class_A, order_id, someclassafield)
(Class_B, order_id, someclassbfield)
(Class_B, order_id, someclassbfield)
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