通过ManyToManyField = Value订购django查询集

Question

If have some models like:

class Tag(models.Model):
    name = models.CharField()

class Thing(models.Model):
    title = models.CharField()
    tags = models.ManyToManyField(Tag)

I can do a filter:

Thing.objects.filter(tags__name='foo')
Thing.objects.filter(tags__name__in=['foo', 'bar'])

But is it possible to order a queryset on the tags value?

Thing.objects.order_by(tags__name='foo')
Thing.objects.order_by(tags__name__in=['foo','bar'])

What I would expect (or like) back in this example, would be ALL Thing models, but ordered where they have a Tag/Tags that I know. I don't want to filter them out, but bring them to the top.

I gather this is possible using the FIELD operator, but seemingly I can only make it work on columns in that models table, eg title, but not on linked tables.

Thanks!

EDIT: After having accepted the below solution, I realised a bug/limitation with it.

If a particular Thing has multiple Tags, then (due to the left join done behind the scenes in the SQL) it will produce one entry for that Thing, for each Tag that it has. With a True or False for each Tag that matches or not.

Adding .distinct() to the queryset helps only slightly, limiting to a max of 2 rows per Thing (ie one tagged=True, and one tagged=False).

I know what I need to do in the SQL, which is to MAX() the CASE(), and then GROUP BY Thing's primary key, which means I will get one row per Thing, and if there has been any tag matches, tagged will be True (and False otherwise).

I see the way that people typically achieve this kind of thing is to use .values() like this:

Thing.objects.values('pk').annotate(tagged=Max(Case(...)))

But the result is only pk and tagged, I need the whole Thing model as the result. So I've managed to achieve what I want, thusly:

from django.db.models import Case, When, Max, BooleanField

tags = ['music'] # for example

queryset = Thing.objects.all().annotate(tagged=Max(Case(
    When(tags__name__in=tags, then=True),
    default=False,
    output_field=BooleanField()
)))
queryset.query.group_by = ['pk']
queryset.order_by('-tagged')

This seems to work, but the group by mechanism feels weird/hacky. Is it acceptable/reliable to group in this way?

Sorry for the epic updated :(

Answer 1

I'd try annotate the query with the conditional value that turns true when the tag is in the list you provide

from django.db.models import Case, When, IntegerField

Thing.objects.annotate(tag_is_known=Case(
    When(tags__name__in=['foo', 'bar'], then=1),
    default=0,
    output_field=IntegerField()
))

Next we use that annotation we called tag_is_known to sort with order_by() :

Thing.objects.annotate(tag_is_known=...).order_by('tag_is_known')

Boolean version

Thing.objects.annotate(tag_is_known=Case(
    When(tags__name__in=['foo', 'bar'], then=True),
    default=False,
    output_field=BooleanField()
))