Oh, sorry, i supposed thaty contains_eager only worked for relationships,
and not for other kind of attributes :D
That's great!
Thanks!
2013/1/28 Michael Bayer <mike...@zzzcomputing.com>
> what additional behaviors are you looking for that the @property approach
> isn't giving you ? Options include turning it into a synonym() which
> would allow it to be a little bit more transparent.
>
>
>
>
>
> On Jan 28, 2013, at 4:55 AM, Pau Tallada wrote:
>
> Hi,
>
> Sorry to insist on this :P
> But there is any way to get this kind of behaviour, the "virtual"
> column/relationship?
>
> Thanks!
>
> Pau.
>
> 2013/1/24 Pau Tallada <tall...@pic.es>
>
>> Hi Michael,
>>
>> As always, thank you very much for your dedication :)
>> I recognize that I am always trying to reach the edges of your wonderful
>> library.
>>
>> That example is just a sample of what I am trying to accomplish:
>> Some sort of "virtual" column/relationship, that it is not mapped to any
>> column, but it can be batch-loaded in a set of instances using a
>> carefully-crafted contains_eager.
>> That way, using a descriptor, if the attribute is not in __dict__ (as you
>> just pointed), i can craft a CTE to fetch the attribute.
>> Or, if the user knows better, will use a with_transformation on top of
>> his query to fetch the attribute for all the queried instances.
>>
>> Thank you again!
>>
>>
>> 2013/1/23 Michael Bayer <mike...@zzzcomputing.com>
>>
>>>
>>> On Jan 23, 2013, at 4:28 PM, Pau Tallada wrote:
>>>
>>> Hi,
>>>
>>> I have prepared a sample of code to ilustrate this behaviour :)
>>>
>>> Summarizing:
>>>
>>> session.expunge_all()
>>> n2 =
>>> session.query(Node).filter_by(name='n2').options(joinedload('ancestors')).one()
>>> print n2.ancestors
>>>
>>>
>>> AND:
>>>
>>> session.expunge_all()
>>> n2 = session.query(Node).filter_by(name='n2').one()
>>> n2 =
>>> session.query(Node).filter_by(name='n2').options(joinedload('ancestors')).one()
>>> print n2.ancestors
>>>
>>>
>>> do NOT produce the same result when the collection is lazy='noload'.
>>>
>>> In the latter, the collection is empty, even after we fetch its contents
>>> with a joinedload.
>>>
>>>
>>> Ah well without actually running it, I can say that the joinedload for
>>> the second case won't have an effect like that, since "n2.ancestors" is
>>> already "loaded", as the effect of "noload" is to populate the attribute
>>> with an empty collection (or None for a scalar). "noload" is not very
>>> widely used and was sort of an added-in strategy from very early versions.
>>>
>>> What it appears like you're looking for on "anscestors" is just the
>>> default "lazy" loader. Above, when you load "n2" without any options,
>>> n2.anscestors will not be present in n2.__dict__ at all, and that's what
>>> "unloaded" means. Hitting it again with the joinedload() should populate
>>> the collection.
>>>
>>> But there's not really a built in system of, "don't load this mapped
>>> attribute, but also don't load anything when touched". So if you stuck
>>> with "noload", you'd have to call expire() on "ancestors" first in order to
>>> get a subsequent loader to populate it.
>>>
>>> I guess if I really wanted an attribute like this, I'd need to wrap it
>>> under a @property:
>>>
>>> @property
>>> def ancestors(self):
>>> if "_ancestors" in self.__dict__:
>>> return self._anscestors
>>> else:
>>> return None
>>>
>>> if I think of a better way I'll let you know.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> 2013/1/23 Pau Tallada <tall...@pic.es>
>>>
>>>> Ok, thank you very much :D
>>>>
>>>> I'll keep poking, and i'll try to provide a sample code :P
>>>>
>>>>
>>>> 2013/1/23 Michael Bayer <mike...@zzzcomputing.com>
>>>>
>>>>>
>>>>> On Jan 23, 2013, at 6:49 AM, Pau Tallada wrote:
>>>>>
>>>>> >
>>>>> > One final comment. With the contains_eager query, the instances that
>>>>> are part of the collection are retrieved from the database but not stored
>>>>> on the identity_map. Is that also the expected behaviour, isn't it?
>>>>>
>>>>> I need to read your whole email more closely but on this point, that's
>>>>> not true at all. Every object that the Session loads from the database
>>>>> goes directly to the identity map. The identity map is weak
>>>>> referencing,
>>>>> so if no other references remain to a particular object, it will disappear
>>>>> from the identity map also, but that implies that object is not accessible
>>>>> anywhere else either.
>>>>>
>>>>> Since it seems like you've been poking around, you might want to
>>>>> continue poking around some more given that information until I have time
>>>>> to look at the rest of your email.
>>>>>
>>>>>
>>>>> --
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>>>>> For more options, visit this group at
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>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> ----------------------------------
>>>> Pau Tallada Crespí
>>>> Dep. d'Astrofísica i Cosmologia
>>>> Port d'Informació Científica (PIC)
>>>> Tel: +34 93 586 8233
>>>> ----------------------------------
>>>>
>>>>
>>>
>>>
>>> --
>>> ----------------------------------
>>> Pau Tallada Crespí
>>> Dep. d'Astrofísica i Cosmologia
>>> Port d'Informació Científica (PIC)
>>> Tel: +34 93 586 8233
>>> ----------------------------------
>>>
>>>
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>>> <noload.py>
>>>
>>>
>>>
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>>>
>>
>>
>>
>> --
>> ----------------------------------
>> Pau Tallada Crespí
>> Dep. d'Astrofísica i Cosmologia
>> Port d'Informació Científica (PIC)
>> Tel: +34 93 586 8233
>> ----------------------------------
>>
>>
>
>
> --
> ----------------------------------
> Pau Tallada Crespí
> Dep. d'Astrofísica i Cosmologia
> Port d'Informació Científica (PIC)
> Tel: +34 93 586 8233
> ----------------------------------
>
>
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--
----------------------------------
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Dep. d'Astrofísica i Cosmologia
Port d'Informació Científica (PIC)
Tel: +34 93 586 8233
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