Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Илья]

>From my subjective point of view `one` is more short and concise, but your
points are valid too, so something like `qs.first(ordered=False)` seems to
be a reasonable tradeoff.

I'd say that its arg should be keyword only to be explicit.

And this arg doesn't then need to be added to `last`.

вс, 14 апр. 2019 г., 13:03 Florian Apolloner :

> Hi,
>
> while your arguments for adding `.one` are all good and well, I think
> adding yet another function will add quite some confusion and will make
> arguing harder imo. As a middleground I could imagine adding an
> `ordered=True/False` argument to first to turn off ordering as needed. This
> will also make it easier to argue about it in the docs I think. Would that
> work?
>
> Cheers,
> Florian
>
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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Curtis Maloney]

On 4/14/19 8:02 PM, Florian Apolloner wrote:

Hi,

while your arguments for adding `.one` are all good and well, I think 
adding yet another function will add quite some confusion and will make 
arguing harder imo. As a middleground I could imagine adding an 
`ordered=True/False` argument to first to turn off ordering as needed. 
This will also make it easier to argue about it in the docs I think. 
Would that work?




I was thinking the same for most of this thread... I would just 
bike-shed it as, for instance "ensure_ordering=False" or 
"stable_order=False" so you more clearly know what you're asking for.


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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Florian Apolloner]

Hi,

while your arguments for adding `.one` are all good and well, I think 
adding yet another function will add quite some confusion and will make 
arguing harder imo. As a middleground I could imagine adding an 
`ordered=True/False` argument to first to turn off ordering as needed. This 
will also make it easier to argue about it in the docs I think. Would that 
work?

Cheers,
Florian

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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[alTus]

Hi. Thank you for that detail response.

It needs to be clarified that I'm not that worried about exception as it 
could seem :)
It was just an answer to those 2 comments to show that their workarounds 
result in different behaviour than proposed `one`.

`get_or_none` would be useful in my opinion too but still it does not 
exactly the same thing because it would still raise MultipleObjectsReturned.

On the other hand, `one` is:
1) totally safe: None for nothing, same as `get` for 1 object, some random 
row if multiple found
2) have the same DB perfomance as `get`
3) has no overhead for exception (can affect performance in long cycles)


воскресенье, 14 апреля 2019 г., 11:23:17 UTC+3 пользователь Shai Berger 
написал:
>
> Hi, 
>
> I agree that first() should imply ordering, and bemoan the decision, 
> made several years ago, that first() should be the answer to all the 
> people asking for get_or_none(). 
>
> That said, get_or_none()'s definition is trivial: 
>
> def get_or_none(qs. *args, **kw): 
> try: 
> return qs.get(*args, **kw) 
> except ObjectDoesNotExist: 
> return None 
>
> and it's not hard to add it as a method on your own querysets or even 
> monkeypatch it into Django's using a descriptor: 
>
> class MonkeyPatcher: 
> def __init__(self, method): 
> self.method = method 
> def __get__(self, instance, cls): 
> return functools.partial(self.method, instance) 
>
> QuerySet.get_or_none = MonkeyPatcher(get_or_none) 
>
> (all above untested, Caveat Lector). 
>
> On a side note, you seem to be very worried about the possibility that 
> an exception will be raised. It may be an issue in terms of flow 
> control, but not in terms of overhead, so including code to handle an 
> exception does solve that issue. 
>
> HTH, 
> Shai. 
>

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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Shai Berger]

Hi,

I agree that first() should imply ordering, and bemoan the decision,
made several years ago, that first() should be the answer to all the
people asking for get_or_none().

That said, get_or_none()'s definition is trivial:

def get_or_none(qs. *args, **kw):
try:
return qs.get(*args, **kw)
except ObjectDoesNotExist:
return None

and it's not hard to add it as a method on your own querysets or even
monkeypatch it into Django's using a descriptor:

class MonkeyPatcher:
def __init__(self, method):
self.method = method
def __get__(self, instance, cls):
return functools.partial(self.method, instance)

QuerySet.get_or_none = MonkeyPatcher(get_or_none)

(all above untested, Caveat Lector).

On a side note, you seem to be very worried about the possibility that
an exception will be raised. It may be an issue in terms of flow
control, but not in terms of overhead, so including code to handle an
exception does solve that issue.

HTH,
Shai.

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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Илья]

First/last enforces ordering because of its nature and also we can't break
compatibility. And second factor is that qs[0] as I wrote above can raise
exception and also will still have ordering (from model.meta for example).

вс, 14 апр. 2019 г., 0:04 Florian Apolloner :

> On Saturday, April 13, 2019 at 10:58:24 PM UTC+2, alTus wrote:
>>
>> I've posted the source code if `first`. You can see there that if qs is
>> not ordered then ordering by pk is added.
>> It's the main point of this issue btw.
>>
>
> Oh I didn't realize that first enforces ordering. Yet another function is
> not a good idea though I fear. I'd check what the reason for enforcing
> ordering in `.first` are and maybe we can just drop that. qs[0] wouldn't
> enforce ordering either (or does it), so why would first do that…
>
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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Joshua Lawes]

If you just want one object that matches your filters during debugging, without 
overhead, why don’t you use .last() instead?

> On 14 Apr 2019, at 7:14 am, Tom Forbes  wrote:
> 
> I don’t think we should add a method like this for a few reasons. Firstly 
> without an order by in SQL the order of rows is undefined, in practice myql 
> orders rows but Postgres returns a different order per transaction. This will 
> be confusing to users who don’t understand this and come to implicitly rely 
> on first() being stable.
> 
> Secondly if you are filtering on an indexed column the overhead will be next 
> to none. Is this not the case?
> 
> And lastly, changing this would be major backwards incompatible change.
> 
> Tom
> 
>> On 13 Apr 2019, at 21:58, alTus  wrote:
>> 
>> I've posted the source code if `first`. You can see there that if qs is not 
>> ordered then ordering by pk is added.
>> It's the main point of this issue btw.
>> 
>> суббота, 13 апреля 2019 г., 22:53:52 UTC+3 пользователь Florian Apolloner 
>> написал:
>>> 
>>> qs.order_by().first() should do what you want and is not worth adding .one()
>>> 
>>> Cheers,
>>> Florian
>>> 
 On Saturday, April 13, 2019 at 5:48:29 PM UTC+2, alTus wrote:
 Hello.
 
 Please consider if that proposal can be useful not only for me :)
 
 `QuerySet.first` is quite useful shortcut but its drawback is that it 
 involves ordering so some DB queries become expensive.
 But quite often (and very often while debugging) there's a need just to 
 get one object that satisfy some filters.
 
 Current `first` implementation to compare with:
 
 def first(self):
 """Return the first object of a query or None if no match is 
 found."""
 for obj in (self if self.ordered else self.order_by('pk'))[:1]:
 return obj
 
 Proposal (as an example of implementation):
 
 def one(self):
 """Return one random object of a query or None if no match is 
 found."""
 for obj in self.order_by()[:1]:
 return obj
 
 That would be short, simple and wouldn't require any imports in client 
 code.
 
 Thank you.
 
>> 
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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Tom Forbes]

I don’t think we should add a method like this for a few reasons. Firstly 
without an order by in SQL the order of rows is undefined, in practice myql 
orders rows but Postgres returns a different order per transaction. This will 
be confusing to users who don’t understand this and come to implicitly rely on 
first() being stable.

Secondly if you are filtering on an indexed column the overhead will be next to 
none. Is this not the case?

And lastly, changing this would be major backwards incompatible change.

Tom

> On 13 Apr 2019, at 21:58, alTus  wrote:
> 
> I've posted the source code if `first`. You can see there that if qs is not 
> ordered then ordering by pk is added.
> It's the main point of this issue btw.
> 
> суббота, 13 апреля 2019 г., 22:53:52 UTC+3 пользователь Florian Apolloner 
> написал:
>> 
>> qs.order_by().first() should do what you want and is not worth adding .one()
>> 
>> Cheers,
>> Florian
>> 
>>> On Saturday, April 13, 2019 at 5:48:29 PM UTC+2, alTus wrote:
>>> Hello.
>>> 
>>> Please consider if that proposal can be useful not only for me :)
>>> 
>>> `QuerySet.first` is quite useful shortcut but its drawback is that it 
>>> involves ordering so some DB queries become expensive.
>>> But quite often (and very often while debugging) there's a need just to get 
>>> one object that satisfy some filters.
>>> 
>>> Current `first` implementation to compare with:
>>> 
>>> def first(self):
>>> """Return the first object of a query or None if no match is 
>>> found."""
>>> for obj in (self if self.ordered else self.order_by('pk'))[:1]:
>>> return obj
>>> 
>>> Proposal (as an example of implementation):
>>> 
>>> def one(self):
>>> """Return one random object of a query or None if no match is 
>>> found."""
>>> for obj in self.order_by()[:1]:
>>> return obj
>>> 
>>> That would be short, simple and wouldn't require any imports in client code.
>>> 
>>> Thank you.
>>> 
> 
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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Florian Apolloner]

On Saturday, April 13, 2019 at 10:58:24 PM UTC+2, alTus wrote:
>
> I've posted the source code if `first`. You can see there that if qs is 
> not ordered then ordering by pk is added.
> It's the main point of this issue btw.
>

Oh I didn't realize that first enforces ordering. Yet another function is 
not a good idea though I fear. I'd check what the reason for enforcing 
ordering in `.first` are and maybe we can just drop that. qs[0] wouldn't 
enforce ordering either (or does it), so why would first do that…

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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[alTus]

I've posted the source code if `first`. You can see there that if qs is not 
ordered then ordering by pk is added.
It's the main point of this issue btw.

суббота, 13 апреля 2019 г., 22:53:52 UTC+3 пользователь Florian Apolloner 
написал:
>
> qs.order_by().first() should do what you want and is not worth adding 
> .one()
>
> Cheers,
> Florian
>
> On Saturday, April 13, 2019 at 5:48:29 PM UTC+2, alTus wrote:
>>
>> Hello.
>>
>> Please consider if that proposal can be useful not only for me :)
>>
>> `QuerySet.first` is quite useful shortcut but its drawback is that it 
>> involves ordering so some DB queries become expensive.
>> But quite often (and very often while debugging) there's a need just to 
>> get one object that satisfy some filters.
>>
>> Current `first` implementation to compare with:
>>
>> def first(self):
>> """Return the first object of a query or None if no match is 
>> found."""
>> for obj in (self if self.ordered else self.order_by('pk'))[:1]:
>> return obj
>>
>> Proposal (as an example of implementation):
>>
>> def one(self):
>> """Return one random object of a query or None if no match is 
>> found."""
>> for obj in self.order_by()[:1]:
>> return obj
>>
>> That would be short, simple and wouldn't require any imports in client 
>> code.
>>
>> Thank you.
>>
>>

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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Florian Apolloner]

qs.order_by().first() should do what you want and is not worth adding .one()

Cheers,
Florian

On Saturday, April 13, 2019 at 5:48:29 PM UTC+2, alTus wrote:
>
> Hello.
>
> Please consider if that proposal can be useful not only for me :)
>
> `QuerySet.first` is quite useful shortcut but its drawback is that it 
> involves ordering so some DB queries become expensive.
> But quite often (and very often while debugging) there's a need just to 
> get one object that satisfy some filters.
>
> Current `first` implementation to compare with:
>
> def first(self):
> """Return the first object of a query or None if no match is 
> found."""
> for obj in (self if self.ordered else self.order_by('pk'))[:1]:
> return obj
>
> Proposal (as an example of implementation):
>
> def one(self):
> """Return one random object of a query or None if no match is 
> found."""
> for obj in self.order_by()[:1]:
> return obj
>
> That would be short, simple and wouldn't require any imports in client 
> code.
>
> Thank you.
>
>

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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[alTus]

No, because qs[0] will raise exception if nothing found (and still it will 
have ordering).

суббота, 13 апреля 2019 г., 21:24:30 UTC+3 пользователь Adam Johnson 
написал:
>
> Doesn’t it work to do qs[0] ?
>
> On Sat, 13 Apr 2019 at 17:48, alTus > 
> wrote:
>
>> Hello.
>>
>> Please consider if that proposal can be useful not only for me :)
>>
>> `QuerySet.first` is quite useful shortcut but its drawback is that it 
>> involves ordering so some DB queries become expensive.
>> But quite often (and very often while debugging) there's a need just to 
>> get one object that satisfy some filters.
>>
>> Current `first` implementation to compare with:
>>
>> def first(self):
>> """Return the first object of a query or None if no match is 
>> found."""
>> for obj in (self if self.ordered else self.order_by('pk'))[:1]:
>> return obj
>>
>> Proposal (as an example of implementation):
>>
>> def one(self):
>> """Return one random object of a query or None if no match is 
>> found."""
>> for obj in self.order_by()[:1]:
>> return obj
>>
>> That would be short, simple and wouldn't require any imports in client 
>> code.
>>
>> Thank you.
>>
>>
>> -- 
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>>  
>> 
>> .
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>>
> -- 
> Adam
>

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Re: Add to QuerySet method `one` that acts as `first` but without ordering

[Adam Johnson]

Doesn’t it work to do qs[0] ?

On Sat, 13 Apr 2019 at 17:48, alTus  wrote:

> Hello.
>
> Please consider if that proposal can be useful not only for me :)
>
> `QuerySet.first` is quite useful shortcut but its drawback is that it
> involves ordering so some DB queries become expensive.
> But quite often (and very often while debugging) there's a need just to
> get one object that satisfy some filters.
>
> Current `first` implementation to compare with:
>
> def first(self):
> """Return the first object of a query or None if no match is
> found."""
> for obj in (self if self.ordered else self.order_by('pk'))[:1]:
> return obj
>
> Proposal (as an example of implementation):
>
> def one(self):
> """Return one random object of a query or None if no match is
> found."""
> for obj in self.order_by()[:1]:
> return obj
>
> That would be short, simple and wouldn't require any imports in client
> code.
>
> Thank you.
>
>
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-- 
Adam

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