Re: A tricky query in one to many relationship - atleast for me:)

Hi Mike,

Thanks for your response. 

I am aware of chaining filters.

The latest() API in django queryset returns an object and not a queryset. Hence, I cannot add a filter to it. 

I had thought about using chaining filters but couldn't find a way.

On Tuesday, August 30, 2016 at 12:18:56 PM UTC+5:30, Mike Dewhirst wrote:

On 30/08/2016 4:33 PM, Web Architect wrote:
> Hi,
>
> I am looking for an elegant and efficient mechanism to have a query
> filter or a solution for the following one to many relationship model.
> Please note the following is just an illustration of the models - hope
> it should provide what I am looking for:
>
> class A(models.Model):
>
>     name = models.CharField(_("Name"), max_length=255, unique=True)
>
>
> class B(models.Model):
>
>     text = models.CharField(_("Text"), max_length=255, unique=True)
>
>     date_created = models.DateTimeField(_("Date Created"),
>     auto_now_add=True)Â
>
>     Â a =Â models.ForeignKey(A, related_name='b')
>
>
> To get all the instances of B associated with a specific instance of A
> (say 'a'), I could do the following : a.b.all()
>
> The latest instance of B associated with 'a' would be :
> a.b.latest('date_created')
>
> Now I would like to have a list of all instances of A where the latest
> instance of B associated with each instance of A will have the 'text'
> field as 'ABCD'.Â
>
> A.objects.filter(b__text='ABCD') will give all instances of A where
> each instance of A will have atleast one instance of B with 'text' =
> 'ABCD'.
>
> There could be a brute force way of getting a solution for the above
> where in I can do A.objects.filter(b__text='ABCD') Â and then go
> through each instance  of A in a for loop over the queryset and check
> for the latest instance of B for text='ABCD'.

HAve you looked at ...

https://docs.djangoproject.com/en/1.8/topics/db/queries/#chaining-filters


>
> As mentioned earlier, I am looking for an elegant and optimal way (if
> any in Django) for the above query.
>
> Thanks.
>
> Â
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