db.models.functions.Greatest

class Greatest(*expressions, **extra) [source]

New in Django 1.9.

Accepts a list of at least two field names or expressions and returns the greatest value. Each argument must be of a similar type, so mixing text and numbers will result in a database error.

Usage example:

class Blog(models.Model):
    body = models.TextField()
    modified = models.DateTimeField(auto_now=True)

class Comment(models.Model):
    body = models.TextField()
    modified = models.DateTimeField(auto_now=True)
    blog = models.ForeignKey(Blog, on_delete=models.CASCADE)

>>> from django.db.models.functions import Greatest
>>> blog = Blog.objects.create(body='Greatest is the best.')
>>> comment = Comment.objects.create(body='No, Least is better.', blog=blog)
>>> comments = Comment.objects.annotate(last_updated=Greatest('modified', 'blog__modified'))
>>> annotated_comment = comments.get()

annotated_comment.last_updated will be the most recent of blog.modified and comment.modified.

Warning

The behavior of Greatest when one or more expression may be null varies between databases:

PostgreSQL: Greatest will return the largest non-null expression, or null if all expressions are null.
SQLite, Oracle, and MySQL: If any expression is null, Greatest will return null.

The PostgreSQL behavior can be emulated using Coalesce if you know a sensible minimum value to provide as a default.

Links:

https://docs.djangoproject.com/en/1.10/ref/models/database-functions/#django.db.models.functions.Greatest

doc_Django

2016-10-09 18:35:45

Comments