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

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

For django >= 2 there should be some minimal changes. so the output will be like this:

def duplicate(obj, value=None, field=None, duplicate_order=None):
    """
    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.
    duplicate_order is a list of models which specify how
    the duplicate objects are saved. For complex objects
    this can matter. Check to save if objects are being
    saved correctly and if not just pass in related objects
    in the order that they should be saved.
    """
    from django.db.models.deletion import Collector
    from django.db.models.fields.related import ForeignKey

    collector = Collector(using='default')
    collector.collect([obj])
    collector.sort()
    related_models = collector.data.keys()
    data_snapshot = {}
    for key in collector.data.keys():
        data_snapshot.update(
            {key: dict(zip([item.pk for item in collector.data[key]], [item for item in collector.data[key]]))})
    root_obj = None

    # Sometimes it's good enough just to save in reverse deletion order.
    if duplicate_order is None:
        duplicate_order = reversed(related_models)

    for model in duplicate_order:
        # Find all FKs on model that point to a related_model.
        fks = []
        for f in model._meta.fields:
            if isinstance(f, ForeignKey) and f.remote_field.related_model in related_models:
                fks.append(f)
        # Replace each `sub_obj` with a duplicate.
        if model not in collector.data:
            continue
        sub_objects = collector.data[model]
        for obj in sub_objects:
            for fk in fks:
                fk_value = getattr(obj, "%s_id" % fk.name)
                # If this FK has been duplicated then point to the duplicate.
                fk_rel_to = data_snapshot[fk.remote_field.related_model]
                if fk_value in fk_rel_to:
                    dupe_obj = fk_rel_to[fk_value]
                    setattr(obj, fk.name, dupe_obj)
            # Duplicate the object and save it.
            obj.id = None
            if field is not None:
                setattr(obj, field, value)
            obj.save()
            if root_obj is None:
                root_obj = obj
    return root_obj

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)

I haven't tried it in django but python's deepcopy might just work for you

EDIT:

You can define custom copy behavior for your models if you implement functions:

__copy__() and __deepcopy__()