Fixed #4027 - Document how to make copies of model instances

    Entry.objects.all().delete()

.. _topics-db-queries-copy:

Copying model instances

=======================

Although there is no built-in method for copying model instances, it is

possible to easily create new instance with all fields' values copied. In the

simplest case, you can just set ``pk`` to ``None``. Using our blog example::

    blog = Blog(name='My blog', tagline='Blogging is easy')

    blog.save() # post.pk == 1

    blog.pk = None

    blog.save() # post.pk == 2

Things get more complicated if you use inheritance. Consider a subclass of

``Blog``::

    class ThemeBlog(Blog):

        theme = models.CharField(max_length=200)

    django_blog = ThemeBlog(name='Django', tagline='Django is easy', theme = 'python')

    django_blog.save() # django_blog.pk == 3

Due to how inheritance works, you have to set both ``pk`` and ``id`` to None::

    django_blog.pk = None

    django_blog.id = None

    django_blog.save() # django_blog.pk == 4

This process does not copy related objects. If you want to copy relations,

you have to write a little bit more code. In our example, ``Entry`` has a many to many

field to ``Author``::

    entry = Entry.objects.all()[0] # some previous entry

    old_authors = entry.authors.all()

    entry.pk = None

    entry.save()

    entry.authors = old_authors # saves new many2many relations

.. _topics-db-queries-update:

Updating multiple objects at once