Django model get unique foreign key rows based on other column max value

问题

I'm new to Django, and I'm trying to create a simple application that keep track on different server configurations in a SQlite database. I've created 2 database models:

from django.db import models

class Server(models.Model):
    name = models.CharField(max_length=250)

class Config(models.Model):
    server = models.ForeignKey(Server, on_delete=models.CASCADE)
    configuration = models.CharField(max_length=250)
    config_version = models.IntegerField()

Here are the 2 models sample data:

Server:
| id     | name    |
| ------ | ------  |
| 1      | Server1 |
| 2      | Server2 |
| 3      | Server3 |

Config:
| id     | configuration | config_version | server |
| ------ | ------------- | -------------- | ------ |
| 1      | srv1_cfg1     | 1              | 1      |
| 2      | srv2_cfg1     | 1              | 2      |
| 3      | srv2_cfg2     | 2              | 2      |
| 4      | srv2_cfg3     | 3              | 2      |
| 5      | srv3_cfg1     | 1              | 3      |
| 6      | srv1_cfg2     | 2              | 1      |
| 7      | srv1_cfg3     | 3              | 1      |

I would like to query the Config table, and get only rows with the maximum value of "config_version" field for each server id, like:

Desired result:
| id     | configuration | config_version | serverid | servername |
| ------ | ------------- | -------------- | -------- | ---------- |
| 4      | srv2_cfg3     | 3              | 2        | Server2    |
| 5      | srv3_cfg1     | 1              | 3        | Server3    |
| 7      | srv1_cfg3     | 3              | 1        | Server1    |

I've tried many different options to construct the correct query, but so far I cannot get what I want. My best result is to query the Server table:

Server.objects.annotate(maxver=Max('config__config_version'))

But it seems I cannot get access to the Config table objects, so I guess I need to query the Config table with some filtering?

I can do this with a raw SQL query, but I would strongly prefer to do it the "Django" way. Any help will be much appreciated.

回答1:

After some more struggle with this, I've came with a solution that works for me. I'm sure it is not optimal, but at least seems to works:

from django.db.models import Max, F

s1 = Config.objects.annotate(maxver=Max('server__config__config_version'))
config_list = s1.filter(config_version=F('maxver'))

If there is a better way to do this, I would love to know it.

来源：https://stackoverflow.com/questions/43252706/django-model-get-unique-foreign-key-rows-based-on-other-column-max-value

标签

django

django-models