Re: Hybrid Property request

[Curtis Maloney]

On 17 July 2013 21:18, Jonathan Slenders wrote:

> This is really nice. Both have their use cases, but I'm prefering the
> second when possible, because it's much more flexible and preserves
> database consistency.
> With the same efford you can make the following work:
>
> ProductEntry.objects.filter(lambda p: p.onpurchase_price * p.quantity
> == 5000.0)
>
> Or if you don't like the lambda keyword, extend the Q object. Something
> like this:
>
> ProductEntry.objects.filter(Q(onpurchase_price) * Q(quantity) == 5000.0)
>
>
ISTM it would be more of an extension of ExpressionNode, like F().

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Re: Hybrid Property request

[Jonathan Slenders]

Oh, and denormalisation is not necessarily faster. If you're able to 
express the value of the column as an SQL expression, it's possible to put 
an index on such an expression.


Le mercredi 17 juillet 2013 13:18:16 UTC+2, Jonathan Slenders a écrit :
>
> This is really nice. Both have their use cases, but I'm prefering the 
> second when possible, because it's much more flexible and preserves 
> database consistency.
> With the same efford you can make the following work:
>
> ProductEntry.objects.filter(lambda p: p.onpurchase_price * p.quantity 
> == 5000.0)
>
> Or if you don't like the lambda keyword, extend the Q object. Something 
> like this:
>
> ProductEntry.objects.filter(Q(onpurchase_price) * Q(quantity) == 5000.0)
>
>
> Denormalisation is fine if you often do the same query. It's certainly 
> easier to implement, but I wouldn't encourage it too much.
>
>
>
> Le mercredi 17 juillet 2013 03:15:52 UTC+2, Alex_Gaynor a écrit :
>>
>> I'm going to suggest that a virtual field and a materialized field are 
>> slightly different concepts, and porblaly have different internals. The 
>> former requires some hooks in the ORM, the latter is really just some nice 
>> wrapping around existing hooks.
>>
>> In any event there's probably some overal with the composite fields GSOC 
>> for the virtual field, when we generalize fields to N>=1, I'm hoping that 
>> makes generlaizing them to N=0 easier.
>>
>> Alex
>>
>>
>> On Wed, Jul 17, 2013 at 11:12 AM, Jacob Kaplan-Moss 
>> wrote:
>>
>>> On Tue, Jul 16, 2013 at 8:04 PM, Russell Keith-Magee <
>>> rus...@keith-magee.com> wrote:
>>>
 So - functionally, what you describe can be done. What's missing is 
 someone to polish the concepts into a patch for inclusion into Django's 
 repository. If this is something that interests you - get hacking! :-)

>>>
>>> I'll bet, in fact, that you could do *both* of these things in a 
>>> *single* implementation. A hybrid property (to use your term) with a 
>>> virtual/concrete flag sounds epic. Show me a good patch and I'll help get 
>>> it merged!
>>>
>>> :)
>>>
>>> Jacob
>>>
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>>>  
>>>  
>>>
>>
>>
>>
>> -- 
>> "I disapprove of what you say, but I will defend to the death your right 
>> to say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
>> "The people's good is the highest law." -- Cicero
>> GPG Key fingerprint: 125F 5C67 DFE9 4084
>>  
>

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Re: Hybrid Property request

[Jonathan Slenders]

This is really nice. Both have their use cases, but I'm prefering the 
second when possible, because it's much more flexible and preserves 
database consistency.
With the same efford you can make the following work:

ProductEntry.objects.filter(lambda p: p.onpurchase_price * p.quantity 
== 5000.0)

Or if you don't like the lambda keyword, extend the Q object. Something 
like this:

ProductEntry.objects.filter(Q(onpurchase_price) * Q(quantity) == 5000.0)


Denormalisation is fine if you often do the same query. It's certainly 
easier to implement, but I wouldn't encourage it too much.



Le mercredi 17 juillet 2013 03:15:52 UTC+2, Alex_Gaynor a écrit :
>
> I'm going to suggest that a virtual field and a materialized field are 
> slightly different concepts, and porblaly have different internals. The 
> former requires some hooks in the ORM, the latter is really just some nice 
> wrapping around existing hooks.
>
> In any event there's probably some overal with the composite fields GSOC 
> for the virtual field, when we generalize fields to N>=1, I'm hoping that 
> makes generlaizing them to N=0 easier.
>
> Alex
>
>
> On Wed, Jul 17, 2013 at 11:12 AM, Jacob Kaplan-Moss 
>  > wrote:
>
>> On Tue, Jul 16, 2013 at 8:04 PM, Russell Keith-Magee <
>> rus...@keith-magee.com > wrote:
>>
>>> So - functionally, what you describe can be done. What's missing is 
>>> someone to polish the concepts into a patch for inclusion into Django's 
>>> repository. If this is something that interests you - get hacking! :-)
>>>
>>
>> I'll bet, in fact, that you could do *both* of these things in a *single* 
>> implementation. A hybrid property (to use your term) with a 
>> virtual/concrete flag sounds epic. Show me a good patch and I'll help get 
>> it merged!
>>
>> :)
>>
>> Jacob
>>
>>  -- 
>> You received this message because you are subscribed to the Google Groups 
>> "Django developers" group.
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>> email to django-develop...@googlegroups.com .
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>> django-d...@googlegroups.com
>> .
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>> For more options, visit https://groups.google.com/groups/opt_out.
>>  
>>  
>>
>
>
>
> -- 
> "I disapprove of what you say, but I will defend to the death your right 
> to say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
> "The people's good is the highest law." -- Cicero
> GPG Key fingerprint: 125F 5C67 DFE9 4084
>  

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Re: Hybrid Property request

[Alex Gaynor]

I'm going to suggest that a virtual field and a materialized field are
slightly different concepts, and porblaly have different internals. The
former requires some hooks in the ORM, the latter is really just some nice
wrapping around existing hooks.

In any event there's probably some overal with the composite fields GSOC
for the virtual field, when we generalize fields to N>=1, I'm hoping that
makes generlaizing them to N=0 easier.

Alex


On Wed, Jul 17, 2013 at 11:12 AM, Jacob Kaplan-Moss wrote:

> On Tue, Jul 16, 2013 at 8:04 PM, Russell Keith-Magee <
> russ...@keith-magee.com> wrote:
>
>> So - functionally, what you describe can be done. What's missing is
>> someone to polish the concepts into a patch for inclusion into Django's
>> repository. If this is something that interests you - get hacking! :-)
>>
>
> I'll bet, in fact, that you could do *both* of these things in a *single*
> implementation. A hybrid property (to use your term) with a
> virtual/concrete flag sounds epic. Show me a good patch and I'll help get
> it merged!
>
> :)
>
> Jacob
>
>  --
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>
>
>



-- 
"I disapprove of what you say, but I will defend to the death your right to
say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
"The people's good is the highest law." -- Cicero
GPG Key fingerprint: 125F 5C67 DFE9 4084

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Re: Hybrid Property request

[Jacob Kaplan-Moss]

On Tue, Jul 16, 2013 at 8:04 PM, Russell Keith-Magee <
russ...@keith-magee.com> wrote:

> So - functionally, what you describe can be done. What's missing is
> someone to polish the concepts into a patch for inclusion into Django's
> repository. If this is something that interests you - get hacking! :-)
>

I'll bet, in fact, that you could do *both* of these things in a *single*
implementation. A hybrid property (to use your term) with a
virtual/concrete flag sounds epic. Show me a good patch and I'll help get
it merged!

:)

Jacob

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Re: Hybrid Property request

[Russell Keith-Magee]

On Wed, Jul 17, 2013 at 4:02 AM, Luis Francisco Jesus Masuelli Bonanni <
endless.l...@gmail.com> wrote:

> A feature I would really appreciate (as devs in
> http://stackoverflow.com/questions/12217763/does-django-orm-have-an-equivalent-to-sqlalchemys-hybrid-attribute,
> at least) is the Hybrid Property feature present in SQLAlchemy. They would
> really contribute to DRY principle.
>
> In django this would allow using it as a left-side for the key=value in a
> filter query (in sqlalchemy they filter by . as part of an
> expression ). Example:
>
> class Purchase(models.Model):
> first_name = CharField(...)
> last_name = CharField(...)
> ...
>
> @hybrid_property
> def full_name(self):
> return self.first_name + ' ' + self.full_name
>
> or ...
>
> class ProductEntry(models.Model):
> purchase = ForeignKey(Purchase, related_name='product_entries')
> onpurchase_price = DecimalField(...)
> product = ForeignKey(Product)
> quantity = DecimalField(...)
> ...
>
> @hybrid_property
> def cost(self):
> return self.onpurchase_price * self.quantity
>
>
> ... being those queries valid and working as expected
>
> Person.objects.filter(full_name__ilike='Martin')
> ProductEntry.objects.filter(cost__lt=5000.0) #SQLAlchemy would use
> ProductEntry.cost < 5000.0
>

To dust off an old trope that I haven't used in a while….

… and a pony!

Seriously -- what you've described sounds interesting, and the idea isn't a
new one. Depending on exactly what you're intending, you could be
describing:

 * A denormalized property. That is, an extra column is stored in the
database with the computed value.

 * A virtual property - that is, there's no extra column, but you can use
the property in queries, and it will be rolled out as a computed alias
column in queries.

Both of these have been proposed in the past (ticket #8946 for denormalized
properties, for example). In the meantime, both of these can be done with
the ORM as it stands.

There are a couple of denormalized property implementations out in the
wild; failing that, you can handle it with a normal Django field, populated
in the model's save() method.

Virtual properties can be emulated using an extra() clause in your query.
It's not as elegant as the syntax you describe, but it works.

So - functionally, what you describe can be done. What's missing is someone
to polish the concepts into a patch for inclusion into Django's repository.
If this is something that interests you - get hacking! :-)

Yours,
Russ Magee %-)

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Hybrid Property request

[Luis Francisco Jesus Masuelli Bonanni]

A feature I would really appreciate (as devs in 
http://stackoverflow.com/questions/12217763/does-django-orm-have-an-equivalent-to-sqlalchemys-hybrid-attribute,
 
at least) is the Hybrid Property feature present in SQLAlchemy. They would 
really contribute to DRY principle.

In django this would allow using it as a left-side for the key=value in a 
filter query (in sqlalchemy they filter by . as part of an 
expression ). Example:

class Purchase(models.Model):
first_name = CharField(...)
last_name = CharField(...)
...

@hybrid_property
def full_name(self):
return self.first_name + ' ' + self.full_name

or ...

class ProductEntry(models.Model):
purchase = ForeignKey(Purchase, related_name='product_entries')
onpurchase_price = DecimalField(...)
product = ForeignKey(Product)
quantity = DecimalField(...)
...

@hybrid_property
def cost(self):
return self.onpurchase_price * self.quantity


... being those queries valid and working as expected

Person.objects.filter(full_name__ilike='Martin')
ProductEntry.objects.filter(cost__lt=5000.0) #SQLAlchemy would use 
ProductEntry.cost < 5000.0

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