Django Queryset 用最新的相关对象注释值

Question

I have three models: Brand, Log and Abstract MWModel.

class Log(models.Model):
    app = models.CharField(max_length=20, choices=get_modules())
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey('content_type', 'object_id')
    method = models.CharField(max_length=8, choices=HTTP_METHODS)
    url = models.TextField()
    body = models.TextField(null=True)
    code = models.IntegerField()
    message = models.TextField(null=True)
    created_at = models.DateTimeField(auto_now_add=True)
    
    def __str__(self):
        return self.created_at.isoformat()


class MWModel(models.Model):
    log = GenericRelation(Log)
    created_at = models.DateTimeField(auto_now_add=True)
    modified_at = models.DateTimeField(auto_now=True)
    
    class Meta:
        abstract = True


class Brand(MWModel):
    name = models.CharField(max_length=50)
    status = models.BooleanField(default=True)
    
    def __str__(self):
        return self.name

How can get code field of last Log record created for each Brand?

I have this

Brand.objects.filter(
    merchant=account.merchant,
    identity__app=F('merchant__erp')
).annotate(
    log__last__created_at=Max('log__created_at'),
    log__last__code=Log.objects.filter(
        content_type__model=Brand._meta.model_name,
        object_id=OuterRef('pk')
    ).order_by('-created_at').values('code')[:1]
).values('pk', 'name', 'log__last__created_at', 'log__last__code')

And i need something like this

Brand.objects.filter(
    merchant=account.merchant,
    identity__app=F('merchant__erp')
).annotate(
    log__last__created_at=Max('log__created_at'),
    log__last__code=F('log__code', filter=Q('log__created_at'=F('log__last__created_at')))
).values('pk', 'name', 'log__last__created_at', 'log__last__code')

But I have not found how to code something similar.

I would greatly appreciate your help. Thanks.

Answer 1

This can be achieved via a subquery. You were really close, just missing a Subquery expression.

Brand.objects.annotate(
    log__last__code=Subquery(
        Log.objects.filter(
            content_type__model=Brand._meta.model_name,
            object_id=OuterRef('pk')
        ).order_by('-created_at').values('code')[:1]
    )
)