Django F() expressions Avoiding race conditions
Example
See this Q&A question if you don't know what race conditions are.
The following code may be subject to race conditions :
article = Article.objects.get(pk=69)
article.views_count += 1
article.save()
If views_count is equal to 1337, this will result in such query:
UPDATE app_article SET views_count = 1338 WHERE id=69
If two clients access this article at the same time, what may happen is that the second HTTP request executes Article.objects.get(pk=69) before the first executes article.save(). Thus, both requests will have views_count = 1337, increment it, and save views_count = 1338 to the database, while it should actually be 1339.
To fix this, use an F() expression:
article = Article.objects.get(pk=69)
article.views_count = F('views_count') + 1
article.save()
This, on the other hand, will result in such query:
UPDATE app_article SET views_count = views_count + 1 WHERE id=69
