Duplicating model instances and their related objects in Django / Algorithm for recusrively duplicating an object


Question

I've models for Books, Chapters and Pages. They are all written by a User:

from django.db import models

class Book(models.Model)
    author = models.ForeignKey('auth.User')

class Chapter(models.Model)
    author = models.ForeignKey('auth.User')
    book = models.ForeignKey(Book)

class Page(models.Model)
    author = models.ForeignKey('auth.User')
    book = models.ForeignKey(Book)
    chapter = models.ForeignKey(Chapter)

What I'd like to do is duplicate an existing Book and update it's User to someone else. The wrinkle is I would also like to duplicate all related model instances to the Book - all it's Chapters and Pages as well!

Things get really tricky when look at a Page - not only will the new Pages need to have their author field updated but they will also need to point to the new Chapter objects!

Does Django support an out of the box way of doing this? What would a generic algorithm for duplicating a model look like?

Cheers,

John


Update:

The classes given above are just an example to illustrate the problem I'm having!

1
35
12/19/2015 7:49:08 AM

Accepted Answer

This no longer works in Django 1.3 as CollectedObjects was removed. See changeset 14507

I posted my solution on Django Snippets. It's based heavily on the django.db.models.query.CollectedObject code used for deleting objects:

from django.db.models.query import CollectedObjects
from django.db.models.fields.related import ForeignKey

def duplicate(obj, value, field):
    """
    Duplicate all related objects of `obj` setting
    `field` to `value`. If one of the duplicate
    objects has an FK to another duplicate object
    update that as well. Return the duplicate copy
    of `obj`.  
    """
    collected_objs = CollectedObjects()
    obj._collect_sub_objects(collected_objs)
    related_models = collected_objs.keys()
    root_obj = None
    # Traverse the related models in reverse deletion order.    
    for model in reversed(related_models):
        # Find all FKs on `model` that point to a `related_model`.
        fks = []
        for f in model._meta.fields:
            if isinstance(f, ForeignKey) and f.rel.to in related_models:
                fks.append(f)
        # Replace each `sub_obj` with a duplicate.
        sub_obj = collected_objs[model]
        for pk_val, obj in sub_obj.iteritems():
            for fk in fks:
                fk_value = getattr(obj, "%s_id" % fk.name)
                # If this FK has been duplicated then point to the duplicate.
                if fk_value in collected_objs[fk.rel.to]:
                    dupe_obj = collected_objs[fk.rel.to][fk_value]
                    setattr(obj, fk.name, dupe_obj)
            # Duplicate the object and save it.
            obj.id = None
            setattr(obj, field, value)
            obj.save()
            if root_obj is None:
                root_obj = obj
    return root_obj
16
3/4/2011 4:33:48 PM

Here's an easy way to copy your object.

Basically:

(1) set the id of your original object to None:

book_to_copy.id = None

(2) change the 'author' attribute and save the ojbect:

book_to_copy.author = new_author

book_to_copy.save()

(3) INSERT performed instead of UPDATE

(It doesn't address changing the author in the Page--I agree with the comments regarding re-structuring the models)


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