Django按ID分组，然后选择最大时间戳

Question

It might be a redundant question, but I have tried previous answers from other related topics and still can't figure it out.

I have a table Board_status looks like this (multiple status and timestamp for each board):

time      | board_id | status
-------------------------------
2012-4-5  |        1 | good

2013-6-6  |        1 | not good

2013-6-7  |        1 | alright

2012-6-8  |        2 | good

2012-6-4  |        3 | good

2012-6-10 |        2 | good

Now I want to select all records from Board_status table, group all of them by board_id for distinct board_id, then select the latest status on each board. Basically end up with table like this (only latest status and timestamp for each board):

time      | board_id | status
------------------------------
2013-6-7  |        1 | alright

2012-6-4  |        3 | good

2012-6-10 |        2 | good

I have tried:

b = Board_status.objects.values('board_id').annotate(max=Max('time')).values_list('board_id','max','status')

but doesn't seem like it is working. Still give me more than 1 record per board_id.

Which command should I use in Django to do this?

Answer 1

An update, this is the solution I use. Not the best, but it works for now:

b=[]
a = Board_status.objects.values('board_id').distinct()
for i in range(a.count()):
 b.append(Board_status.objects.filter(board_id=a[i]['board_id']).latest('time'))

So I got all board_id, store into list a. Then for each board_id, do another query to get the latest time. Any better answer is still welcomed.

Answer 2

How will it work? You neither have filter nor distinct to filter out the duplicates. I am not sure if this can be easily done in a single django query. You should read more on:

1. https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.distinct
2. https://docs.djangoproject.com/en/1.4/topics/db/aggregation/

If you can't do it in 1 raw sql query, you can't do it with an OR mapper either as it's built on top of mysql (in your case). Can you tell me how you would do this via raw SQL?