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Mastering Django: Core

You're reading from   Mastering Django: Core The Complete Guide to Django 1.8 LTS

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Product type Paperback
Published in Dec 2016
Publisher
ISBN-13 9781787281141
Length 694 pages
Edition 1st Edition
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Author (1):
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Nigel George Nigel George
Author Profile Icon Nigel George
Nigel George
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Table of Contents (33) Chapters Close

Mastering Django: Core
Credits
About the Author
www.PacktPub.com
Preface
1. Introduction to Django and Getting Started FREE CHAPTER 2. Views and URLconfs 3. Templates 4. Models 5. The Django Admin Site 6. Forms 7. Advanced Views and URLconfs 8. Advanced Templates 9. Advanced Models 10. Generic Views 11. User Authentication in Django 12. Testing in Django 13. Deploying Django 14. Generating Non-HTML Content 15. Django Sessions 16. Djangos Cache Framework 17. Django Middleware 18. Internationalization 19. Security in Django 20. More on Installing Django 21. Advanced Database Management Model Definition Reference Database API Reference Generic View Reference Settings Built-in Template Tags and Filters Request and Response Objects Developing Django with Visual Studio

Updating multiple objects at once


Sometimes you want to set a field to a particular value for all the objects in a QuerySet. You can do this with the update() method. For example:

# Update all the headlines with pub_date in 2007.
Entry.objects.filter(pub_date__year=2007).update(headline='Everything is the same')

You can only set non-relation fields and ForeignKey fields using this method. To update a non-relation field, provide the new value as a constant. To update ForeignKey fields, set the new value to be the new model instance you want to point to. For example:

>>> b = Blog.objects.get(pk=1)
# Change every Entry so that it belongs to this Blog.
>>> Entry.objects.all().update(blog=b)

The update() method is applied instantly and returns the number of rows matched by the query (which may not be equal to the number of rows updated if some rows already have the new value).

The only restriction on the QuerySet that is updated is that it can only access one database table, the model's main table. You can filter based on related fields, but you can only update columns in the model's main table. Example:

>>> b = Blog.objects.get(pk=1)
# Update all the headlines belonging to this Blog.
>>> Entry.objects.select_related().filter(blog=b).update
(headline='Everything is the same')

Be aware that the update() method is converted directly to an SQL statement. It is a bulk operation for direct updates. It doesn't run any save() methods on your models, or emit the pre_save or post_save signals (which are a consequence of calling save()), or honor the auto_now field option. If you want to save every item in a QuerySet and make sure that the save() method is called on each instance, you don't need any special function to handle that. Just loop over them and call save():

for item in my_queryset: 
    item.save() 

Calls to update can also use F expressions to update one field based on the value of another field in the model. This is especially useful for incrementing counters based upon their current value. For example, to increment the pingback count for every entry in the blog:

>>> Entry.objects.all().update(n_pingbacks=F('n_pingbacks') + 1)

However, unlike F() objects in filter and exclude clauses, you can't introduce joins when you use F() objects in an update-you can only reference fields local to the model being updated. If you attempt to introduce a join with an F() object, a FieldError will be raised:

# THIS WILL RAISE A FieldError
>>> Entry.objects.update(headline=F('blog__name'))
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