Django 1.7.1：子模型，删除对父模型的所有引用，并使子模型从爷爷继承

Question

I have three models like:

class A(models.Model):
    pass
    # more fields here

class B(A):
    pass
    # more fields here

class C(B):
    pass
    # more fields here

New requirements require me to instead have:

class A(models.Model):
    pass
    # more fields here

class B(A):
    pass
    # more fields here

class C(A):
    pass
    # more fields here

The code is already running in production, so I need to do some data migration. Thinking that it would work smoothly, here's what I planned to do:

1. Save each C 's b_ptr_id to a temporary field.
2. Remove C 's inheritance from B (deleting b_ptr_id in the process).
3. Make C inherit from A (adding an a_ptr_id in the process).
4. For each C , copy the value stored in the temporary field to a_ptr_id .
5. Delete the temporary field.

My problem is with step 3. When I try to make inherit C from A , I'm getting the error django.core.exceptions.FieldError: Local field 'a_ptr' in class 'C' clashes with field of similar name from base class 'B' . So it seems like Django still "remembers" that C used to inherit from B , even if b_ptr_id is no longer a column of C in the database . How do I make it forget so that I can move on? Or is there a better way to make child models inherit from its grandparent model instead of its parent model?

Answer 1

Not sure if there's an easier way to do this but....

Try creating a C' that inherits A. Then create a program that creates a C' instance of each C instance. (Delete all instance of C afterwards)

If you want to keep the name C, update schema for C and then repopulate C from the C' instances.

Hope it helps. :)