Re: Making ".only()" a bit smarter

[Pablo Recio]

Yeah, you might be right. It just feels weird that if I'm trying to fetch
*only* a FK, it needs to do a second lookup. But explicit is better than
implicit indeed.


On 19 July 2013 21:48, Michal Petrucha  wrote:

> On Fri, Jul 19, 2013 at 01:02:34PM +0100, Pablo Recio wrote:
> > Hi devs,
> >
> > I just came up with the following scenario:
> >
> > class A(models.Model):
> > # some fields...
> > b = models.ForeignKey(B)
> >
> > class B(models.Model):
> > # other fields
> >
> > A simple FK relationship. Well, then I wanted to fetch objects from 'A'
> > model but just for retrieve the related 'B', so I tried something like:
> >
> > A.objects.only('b').all()
> >
> > I hoped it was going to be smart enough to check that I'm accessing to FK
> > and then do 'select_related' for that field behind the scenes, but it
> > doesn't, so I needed to chain the select_related myself.
> >
> > Is there any reason for this behaviour? If not, would it be a good patch
> to
> > submit?
>
> Explicit is better than implicit. Also, special cases aren't special
> enough to break the rules.
>
> To me, what you propose seems too arbitrary. `only` would do what it
> says it does, that is, defer everything but the selected fields,
> except for the special case where you restrict the query to a single
> ForeignKey, in which case it would also do a `select_related` -- that
> sounds like a quite specific special case.
>
> Also, what would you propose to happen in case you do `defer` on all
> fields except one ForeignKey? Would that do the same thing as the
> equivalent `only` or not? Why?
>
> In other words, I don't think it is worth complicating the API in this
> way just to save a single method call on a QuerySet.
>
> Michal
>



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Pablo Recio Quijano

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Re: Making ".only()" a bit smarter

[Michal Petrucha]

On Fri, Jul 19, 2013 at 01:02:34PM +0100, Pablo Recio wrote:
> Hi devs,
> 
> I just came up with the following scenario:
> 
> class A(models.Model):
> # some fields...
> b = models.ForeignKey(B)
> 
> class B(models.Model):
> # other fields
> 
> A simple FK relationship. Well, then I wanted to fetch objects from 'A'
> model but just for retrieve the related 'B', so I tried something like:
> 
> A.objects.only('b').all()
> 
> I hoped it was going to be smart enough to check that I'm accessing to FK
> and then do 'select_related' for that field behind the scenes, but it
> doesn't, so I needed to chain the select_related myself.
> 
> Is there any reason for this behaviour? If not, would it be a good patch to
> submit?

Explicit is better than implicit. Also, special cases aren't special
enough to break the rules.

To me, what you propose seems too arbitrary. `only` would do what it
says it does, that is, defer everything but the selected fields,
except for the special case where you restrict the query to a single
ForeignKey, in which case it would also do a `select_related` -- that
sounds like a quite specific special case.

Also, what would you propose to happen in case you do `defer` on all
fields except one ForeignKey? Would that do the same thing as the
equivalent `only` or not? Why?

In other words, I don't think it is worth complicating the API in this
way just to save a single method call on a QuerySet.

Michal


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