One of the neat things making it's way into Django 1.1 is F object queries. The F object is kind of like the Q object as it can be used it queries but it represents a database field on the right hand side of an equality/inequality.
For the example I'll use the example models from the "Making Queries" section of the Django Documentation.
| class Blog(models.Model): | |
| name = models.CharField(max_length=100) | |
| tagline = models.TextField() | |
| def __unicode__(self): | |
| return self.name | |
| class Author(models.Model): | |
| name = models.CharField(max_length=50) | |
| email = models.EmailField() | |
| def __unicode__(self): | |
| return self.name | |
| class Entry(models.Model): | |
| blog = models.ForeignKey(Blog) | |
| headline = models.CharField(max_length=255) | |
| body_text = models.TextField() | |
| pub_date = models.DateTimeField() | |
| authors = models.ManyToManyField(Author) | |
| n_comments = models.IntegerField() | |
| n_pingbacks = models.IntegerField() | |
| rating = models.IntegerField() | |
| def __unicode__(self): | |
| return self.headline |
Here we can do cool stuff like query for blog entries where the number of comments equals the number of pingbacks.
| >>> from django.db.models import F | |
| >>> Entry.objects.filter(n_pingbacks__lt=F('n_comments')) |
You can perform operations on colums or add columns together.
| >>> Entry.objects.filter(n_pingbacks__lt=F('n_comments') * 2) | |
| >>> Entry.objects.filter(rating__lt=F('n_comments') + F('n_pingbacks')) |
You can even span relationships across tables
| >>> Entry.objects.filter(author__name=F('blog__name')) |
This query ended up like this. ftester is the name of the application I made to test this.
| SELECT `ftester_entry`.`id`, `ftester_entry`.`blog_id`, `ftester_entry`.`headline`, `ftester_entry`.`body_text`, `ftester_entry`.`pub_date`, `ftester_entry`.`n_comments`, `ftester_entry`.`n_pingbacks`, `ftester_entry`.`rating` FROM `ftester_entry` INNER JOIN `ftester_blog` ON (`ftester_entry`.`blog_id` = `ftester_blog`.`id`) INNER JOIN `ftester_entry_authors` ON (`ftester_entry`.`id` = `ftester_entry_authors`.`entry_id`) INNER JOIN `ftester_author` ON (`ftester_entry_authors`.`author_id` = `ftester_author`.`id`) WHERE `ftester_author`.`name` = `ftester_blog`.`name` LIMIT 21 |
Note: As an aside it's interesting to note the limit on this query which actually only gets 21 records. I haven't tested it but I suppose that Django only gets a set of records at a time for performance reasons.
But the reason the F() object was created was to allow using the value of one column in another column during an update. This allows you do do things like add 1 to the pingbacks for every entry in one go without selecting the whole batch and updating the field.
| Entry.objects.all().update(n_pingbacks=F('n_pingbacks') + 1) |