I have a model similar to this:
class Tree(models.Model):
description = models.CharField(max_length = 255, null = False, blank = False)
parent = models.ForeignKey("Tree", null = True, blank = True, on_delete = models.CASCADE)
class Meta:
ordering = ['description', '-id']
I need to find the latest record for each parent.
I tried with this:
latests = Tree.objects.values("parent").annotate(last = Max("pk"))
The result is not correct because the SQL query is:
SELECT parent_id, MAX(id) AS last FROM tree GROUP BY id;
The ORM translates the foreign key to the source and does not use the value inside the field. Is there a way not to "follow" the foreign key and to use instead the value of the field?
The model generated in the PostgreSQL database the table named tree with three columns:
Column | Type
------------+-----------------------
id | integer
description | character varying(255)
parent_id | integer
With this data:
id | description | parent_id
----+-------------+----------
1 | A | 1
2 | B | 2
3 | C | 1
4 | D | 1
5 | E | 2
I want this result:
last | parent_id
-----+----------
5 | 2
4 | 1
I can do this simply in SQL with:
select max(id) as last, parent_id from tree group by parent_id
