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Fixed #4027 - Document how to make copies of model instances
Thanks to Marek Kubica for the report and initial patch, and to oinopion and erikr for work on the patch. git-svn-id: http://code.djangoproject.com/svn/django/trunk@17064 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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@ -828,6 +828,46 @@ complete query set::
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Entry.objects.all().delete()
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.. _topics-db-queries-copy:
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Copying model instances
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=======================
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Although there is no built-in method for copying model instances, it is
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possible to easily create new instance with all fields' values copied. In the
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simplest case, you can just set ``pk`` to ``None``. Using our blog example::
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blog = Blog(name='My blog', tagline='Blogging is easy')
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blog.save() # post.pk == 1
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blog.pk = None
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blog.save() # post.pk == 2
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Things get more complicated if you use inheritance. Consider a subclass of
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``Blog``::
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class ThemeBlog(Blog):
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theme = models.CharField(max_length=200)
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django_blog = ThemeBlog(name='Django', tagline='Django is easy', theme = 'python')
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django_blog.save() # django_blog.pk == 3
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Due to how inheritance works, you have to set both ``pk`` and ``id`` to None::
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django_blog.pk = None
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django_blog.id = None
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django_blog.save() # django_blog.pk == 4
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This process does not copy related objects. If you want to copy relations,
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you have to write a little bit more code. In our example, ``Entry`` has a many to many
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field to ``Author``::
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entry = Entry.objects.all()[0] # some previous entry
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old_authors = entry.authors.all()
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entry.pk = None
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entry.save()
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entry.authors = old_authors # saves new many2many relations
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.. _topics-db-queries-update:
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Updating multiple objects at once
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