Fast and Easy way to check if an instance of an Model exists in the Database
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Johannes Schneider
Jan 16, 2014, 8:27:09 AM1/16/14
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Hi List,
I need to check, if a given instance of a Model already exists in the
Database. I cannot check 'instance.pk is None', because there are
manually given PKs (no discussion about this plz).
Up to now I check if 'instance._state.adding' is true. But I don't like
this solution.
Checking 'InstanceModel.objects.exists(pk=self.pk)' generates too much
queries, so this is not acceptable in my case.
bg
Johannes
--
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
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I need to check, if a given instance of a Model already exists in the
Database. I cannot check 'instance.pk is None', because there are
manually given PKs (no discussion about this plz).
Up to now I check if 'instance._state.adding' is true. But I don't like
this solution.
Checking 'InstanceModel.objects.exists(pk=self.pk)' generates too much
queries, so this is not acceptable in my case.
bg
Johannes
--
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
http://www.galileo-press.de/
Gesch�ftsf�hrer: Tomas Wehren, Ralf Kaulisch, Rainer Kaltenecker
HRB 8363 Amtsgericht Bonn
Erik Cederstrand
Jan 16, 2014, 10:04:22 AM1/16/14
to Django Users
Den 16/01/2014 kl. 14.27 skrev Johannes Schneider <johannes....@galileo-press.de>:
> Hi List,
> I need to check, if a given instance of a Model already exists in the Database. I cannot check 'instance.pk is None', because there are manually given PKs (no discussion about this plz).
>
> Up to now I check if 'instance._state.adding' is true. But I don't like this solution.
> Checking 'InstanceModel.objects.exists(pk=self.pk)' generates too much queries, so this is not acceptable in my case.
Can you elaborate on “too much queries”? exists() is supposed to be the cheapest way of checking if the object exists. It seems like you want to check the database, without actually checking the database…
> Hi List,
> I need to check, if a given instance of a Model already exists in the Database. I cannot check 'instance.pk is None', because there are manually given PKs (no discussion about this plz).
>
> Up to now I check if 'instance._state.adding' is true. But I don't like this solution.
> Checking 'InstanceModel.objects.exists(pk=self.pk)' generates too much queries, so this is not acceptable in my case.
If you want to check a large number of objects at the same time, just do something like:
InstanceModel.objects.filter(id__in=(your, list, of, PKs)).values_list('id', flat=True)
to get all pre-existing PKs in one query.
Erik
Johannes Schneider
Jan 16, 2014, 11:50:39 AM1/16/14
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The point is, I have to distinguish between an instance which is
created, but its save method is not yet called. And an instance coming
from the database via a manager.
bg,
Johannes
On 16.01.2014 16:04, Erik Cederstrand wrote:
> Den 16/01/2014 kl. 14.27 skrev Johannes Schneider <johannes....@galileo-press.de>:
>
>> Hi List,
>> I need to check, if a given instance of a Model already exists in the Database. I cannot check 'instance.pk is None', because there are manually given PKs (no discussion about this plz).
>>
>> Up to now I check if 'instance._state.adding' is true. But I don't like this solution.
>> Checking 'InstanceModel.objects.exists(pk=self.pk)' generates too much queries, so this is not acceptable in my case.
>
> Can you elaborate on �too much queries�? exists() is supposed to be the cheapest way of checking if the object exists. It seems like you want to check the database, without actually checking the database�
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created, but its save method is not yet called. And an instance coming
from the database via a manager.
bg,
Johannes
On 16.01.2014 16:04, Erik Cederstrand wrote:
> Den 16/01/2014 kl. 14.27 skrev Johannes Schneider <johannes....@galileo-press.de>:
>
>> Hi List,
>> I need to check, if a given instance of a Model already exists in the Database. I cannot check 'instance.pk is None', because there are manually given PKs (no discussion about this plz).
>>
>> Up to now I check if 'instance._state.adding' is true. But I don't like this solution.
>> Checking 'InstanceModel.objects.exists(pk=self.pk)' generates too much queries, so this is not acceptable in my case.
>
>
> If you want to check a large number of objects at the same time, just do something like:
>
> InstanceModel.objects.filter(id__in=(your, list, of, PKs)).values_list('id', flat=True)
>
> to get all pre-existing PKs in one query.
>
> Erik
>
> If you want to check a large number of objects at the same time, just do something like:
>
> InstanceModel.objects.filter(id__in=(your, list, of, PKs)).values_list('id', flat=True)
>
> to get all pre-existing PKs in one query.
>
> Erik
>
--
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
http://www.galileo-press.de/
Gesch�ftsf�hrer: Tomas Wehren, Ralf Kaulisch, Rainer Kaltenecker
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
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Martin Marrese
Jan 16, 2014, 11:53:01 AM1/16/14
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On Thu, Jan 16, 2014 at 1:50 PM, Johannes Schneider <johannes....@galileo-press.de> wrote:
The point is, I have to distinguish between an instance which is created, but its save method is not yet called. And an instance coming from the database via a manager.
bg,
Johannes
Perhaps get_or_create (https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create) ?
Martín
Martin Marrese
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+54 911 5112 9448
Javier Guerra Giraldez
Jan 16, 2014, 12:03:51 PM1/16/14
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On Thu, Jan 16, 2014 at 11:50 AM, Johannes Schneider
<johannes....@galileo-press.de> wrote:
> The point is, I have to distinguish between an instance which is created,
> but its save method is not yet called. And an instance coming from the
> database via a manager.
AFAIK, if it has a 'pk' property, it has been saved.
<johannes....@galileo-press.de> wrote:
> The point is, I have to distinguish between an instance which is created,
> but its save method is not yet called. And an instance coming from the
> database via a manager.
--
Javier
Johannes Schneider
Jan 16, 2014, 12:22:36 PM1/16/14
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No, just if you use auto generated PKs. Exactly this is the point. We
use manually generated PKs, so can not use this check.
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use manually generated PKs, so can not use this check.
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
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Geschäftsführer: Tomas Wehren, Ralf Kaulisch, Rainer Kaltenecker
Webentwicklung
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Johannes Schneider
Jan 16, 2014, 12:23:38 PM1/16/14
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This could be a way to it, but we need this information while handling
the Pre_save singal. So this does not work too.
On 16.01.2014 17:53, Martin Marrese wrote:
> On Thu, Jan 16, 2014 at 1:50 PM, Johannes Schneider
> <johannes....@galileo-press.de
>
> --
> Martin Marrese
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Johannes Schneider
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the Pre_save singal. So this does not work too.
On 16.01.2014 17:53, Martin Marrese wrote:
> On Thu, Jan 16, 2014 at 1:50 PM, Johannes Schneider
> <johannes....@galileo-press.de
> <mailto:johannes....@galileo-press.de>> wrote:
>
> The point is, I have to distinguish between an instance which is
> created, but its save method is not yet called. And an instance
> coming from the database via a manager.
>
> bg,
> Johannes
>
>
> Perhaps get_or_create
> (https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create)
> ?
>
> Mart�n
>
> The point is, I have to distinguish between an instance which is
> created, but its save method is not yet called. And an instance
> coming from the database via a manager.
>
> bg,
> Johannes
>
>
> Perhaps get_or_create
> (https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create)
> ?
>
>
> --
> Martin Marrese
> mar...@gmail.com <mailto:mar...@gmail.com>
> +54 911 5112 9448
>
> --
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--
Johannes Schneider
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Erik Cederstrand
Jan 17, 2014, 3:35:40 AM1/17/14
to Django Users
Den 16/01/2014 kl. 17.50 skrev Johannes Schneider <johannes....@galileo-press.de>:
> The point is, I have to distinguish between an instance which is created, but its save method is not yet called. And an instance coming from the database via a manager.
Okay, then create an instance variable on the model instances when you create them manually. Something like this:
> The point is, I have to distinguish between an instance which is created, but its save method is not yet called. And an instance coming from the database via a manager.
obj = InstanceModel()
obj.pk = 123
obj.manually_created = True
Then have your save() method check for this variable:
def save(self, *args, **kwargs):
if getattr(self, ‘manually_created’, False):
# Was created manually. Do something
else:
# From database. Do something else
super(self, InstanceModel).save(*args, **kwargs)
del obj.manually_created
Erik
Tom Evans
Jan 17, 2014, 8:39:36 AM1/17/14
to django...@googlegroups.com
On Thu, Jan 16, 2014 at 5:23 PM, Johannes Schneider
<johannes....@galileo-press.de> wrote:
> This could be a way to it, but we need this information while handling the
> Pre_save singal. So this does not work too.
Abuse the django model instance's internals and look to see what
<johannes....@galileo-press.de> wrote:
> This could be a way to it, but we need this information while handling the
> Pre_save singal. So this does not work too.
database this instance was loaded from - if it is None, it hasn't come
from the database.
This is highly fragile - using an undocumented API that I know has
changed in recent versions. Personally I would test that the pk does
not exist in the database (and buy faster hardware.)
>>> u=User.objects.get(id=1)
>>> u2=User()
>>> u._state.db
'default'
>>> u2._state.db
>>>
Cheers
Tom
Johannes Schneider
Jan 20, 2014, 3:12:43 AM1/20/14
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thnx for this suggestion. we use '_state.adding' now. Is there an
advantage of using '_state.database'?
bg,
Johannes
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advantage of using '_state.database'?
bg,
Johannes
--
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
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Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
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Geschäftsführer: Tomas Wehren, Ralf Kaulisch, Rainer Kaltenecker
Johannes Schneider
Webentwicklung
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Kelly Nicholes
Jan 20, 2014, 10:25:33 AM1/20/14
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Wait-- Why aren't you using exists()? Don't even check the pk. Model.query.exists() was made for this.
Johannes Schneider
Jan 20, 2014, 11:13:15 AM1/20/14
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I don't do, because I don't want do to have a query against the database.
On 20.01.2014 16:25, Kelly Nicholes wrote:
> Wait-- Why aren't you using exists()? Don't even check the pk. Model.query.exists() was made for this.
>
On 20.01.2014 16:25, Kelly Nicholes wrote:
> Wait-- Why aren't you using exists()? Don't even check the pk. Model.query.exists() was made for this.
>
Avraham Serour
Jan 20, 2014, 12:21:23 PM1/20/14
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well you have a problem then, the database is the one holding the data, if you want to know if the data exists will would need to ask the one responsible for it, if not the DB then who?
you could keep a copy in memory but then you will have two problems. are you actually having problems with exists() or is this just premature optimization?
On Mon, Jan 20, 2014 at 6:13 PM, Johannes Schneider <johannes....@galileo-press.de> wrote:
I don't do, because I don't want do to have a query against the database.
On 20.01.2014 16:25, Kelly Nicholes wrote:
Wait-- Why aren't you using exists()? Don't even check the pk. Model.query.exists() was made for this.
--
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Johannes Schneider
Jan 21, 2014, 2:58:33 AM1/21/14
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maybe I I formulated it not precise enough.
I need to check if some instance of a model comes from some database
query (e.g. via get(..)) or is instanciated 'by hand'. and not jet
written to the database.
In this case I don't need to know, if there is an related object (e.g.
with the same pk or same values for its attributes) in the database.
bg,
Johannes
On 20.01.2014 18:21, Avraham Serour wrote:
> well you have a problem then, the database is the one holding the data,
> if you want to know if the data exists will would need to ask the one
> responsible for it, if not the DB then who?
>
> you could keep a copy in memory but then you will have two problems. are
> you actually having problems with exists() or is this just premature
> optimization?
>
>
> On Mon, Jan 20, 2014 at 6:13 PM, Johannes Schneider
> <johannes....@galileo-press.de
> <mailto:johannes....@galileo-press.de>
> Tel.: +49.228.42150 <tel:%2B49.228.42150>.xxx
> http://www.galileo-press.de/
>
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>
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I need to check if some instance of a model comes from some database
query (e.g. via get(..)) or is instanciated 'by hand'. and not jet
written to the database.
In this case I don't need to know, if there is an related object (e.g.
with the same pk or same values for its attributes) in the database.
bg,
Johannes
On 20.01.2014 18:21, Avraham Serour wrote:
> well you have a problem then, the database is the one holding the data,
> if you want to know if the data exists will would need to ask the one
> responsible for it, if not the DB then who?
>
> you could keep a copy in memory but then you will have two problems. are
> you actually having problems with exists() or is this just premature
> optimization?
>
>
> On Mon, Jan 20, 2014 at 6:13 PM, Johannes Schneider
> <johannes....@galileo-press.de
> <mailto:johannes....@galileo-press.de>> wrote:
>
> I don't do, because I don't want do to have a query against the
> database.
>
>
> On 20.01.2014 16:25, Kelly Nicholes wrote:
>
> Wait-- Why aren't you using exists()? Don't even check the pk.
> Model.query.exists() was made for this.
>
>
>
> --
> Johannes Schneider
> Webentwicklung
> johannes.schneider@galileo-__press.de
>
> I don't do, because I don't want do to have a query against the
> database.
>
>
> On 20.01.2014 16:25, Kelly Nicholes wrote:
>
> Wait-- Why aren't you using exists()? Don't even check the pk.
> Model.query.exists() was made for this.
>
>
>
> --
> Johannes Schneider
> Webentwicklung
> <mailto:johannes....@galileo-press.de>
> Tel.: +49.228.42150 <tel:%2B49.228.42150>.xxx
>
> Galileo Press GmbH
> Rheinwerkallee 4 - 53227 Bonn - Germany
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> http://www.galileo-press.de/
>
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Erik Cederstrand
Jan 21, 2014, 3:16:22 AM1/21/14
to Django Users
Den 21/01/2014 kl. 08.58 skrev Johannes Schneider <johannes....@galileo-press.de>:
> maybe I I formulated it not precise enough.
> I need to check if some instance of a model comes from some database query (e.g. via get(..)) or is instanciated 'by hand'. and not jet written to the database.
> In this case I don't need to know, if there is an related object (e.g. with the same pk or same values for its attributes) in the database.
Okay. Sounds like your best bet is to either roll your own, or use the undocumented API as suggested by Tom Evans.
> maybe I I formulated it not precise enough.
> I need to check if some instance of a model comes from some database query (e.g. via get(..)) or is instanciated 'by hand'. and not jet written to the database.
> In this case I don't need to know, if there is an related object (e.g. with the same pk or same values for its attributes) in the database.
Either way, make sure to write tests so this doesn’t break when you upgrade Django.
Erik
Avraham Serour
Jan 21, 2014, 4:07:28 AM1/21/14
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wouldn't checking for the presence of an id enough? as far as I know instantiated objects don't have id's
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Johannes Schneider
Jan 21, 2014, 4:55:55 AM1/21/14
to django...@googlegroups.com
yes, but the problem is that we manually give them a pk.
Maye it's the best to reduce the question to the following:
Which (undocumented) field is better for this case?
* instance._state.db (checking if this is None)
* instance_state.adding (checking for True)
bg,
Johannes
On 21.01.2014 10:07, Avraham Serour wrote:
> wouldn't checking for the presence of an id enough? as far as I know
> instantiated objects don't have id's
>
>
> On Tue, Jan 21, 2014 at 10:16 AM, Erik Cederstrand
> <erik+...@cederstrand.dk <mailto:erik+...@cederstrand.dk>> wrote:
>
> Den 21/01/2014 kl. 08.58 skrev Johannes Schneider
> <johannes....@galileo-press.de
> <mailto:johannes....@galileo-press.de>>:
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Maye it's the best to reduce the question to the following:
Which (undocumented) field is better for this case?
* instance._state.db (checking if this is None)
* instance_state.adding (checking for True)
bg,
Johannes
On 21.01.2014 10:07, Avraham Serour wrote:
> wouldn't checking for the presence of an id enough? as far as I know
> instantiated objects don't have id's
>
>
> On Tue, Jan 21, 2014 at 10:16 AM, Erik Cederstrand
> <erik+...@cederstrand.dk <mailto:erik+...@cederstrand.dk>> wrote:
>
> Den 21/01/2014 kl. 08.58 skrev Johannes Schneider
> <johannes....@galileo-press.de
>
> > maybe I I formulated it not precise enough.
> > I need to check if some instance of a model comes from some
> database query (e.g. via get(..)) or is instanciated 'by hand'. and
> not jet written to the database.
> > In this case I don't need to know, if there is an related object
> (e.g. with the same pk or same values for its attributes) in the
> database.
>
> Okay. Sounds like your best bet is to either roll your own, or use
> the undocumented API as suggested by Tom Evans.
>
> Either way, make sure to write tests so this doesn�t break when you
> > maybe I I formulated it not precise enough.
> > I need to check if some instance of a model comes from some
> database query (e.g. via get(..)) or is instanciated 'by hand'. and
> not jet written to the database.
> > In this case I don't need to know, if there is an related object
> (e.g. with the same pk or same values for its attributes) in the
> database.
>
> Okay. Sounds like your best bet is to either roll your own, or use
> the undocumented API as suggested by Tom Evans.
>
> upgrade Django.
>
> Erik
>
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> Erik
>
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Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
http://www.galileo-press.de/
Gesch�ftsf�hrer: Tomas Wehren, Ralf Kaulisch, Rainer Kaltenecker
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
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Avraham Serour
Jan 21, 2014, 6:59:20 AM1/21/14
to django...@googlegroups.com
pk is not necessarily id, in any case if you are the one adding it then you already know that the object didn't come from the db, in order to check that later I would add an attribute to the object and check it's presence, if not present it means it came from the db.
On Tue, Jan 21, 2014 at 11:55 AM, Johannes Schneider <johannes....@galileo-press.de> wrote:
yes, but the problem is that we manually give them a pk.
Maye it's the best to reduce the question to the following:
Which (undocumented) field is better for this case?
* instance._state.db (checking if this is None)
* instance_state.adding (checking for True)
bg,
Johannes
On 21.01.2014 10:07, Avraham Serour wrote:
wouldn't checking for the presence of an id enough? as far as I know
instantiated objects don't have id's
On Tue, Jan 21, 2014 at 10:16 AM, Erik Cederstrand
<erik+...@cederstrand.dk <mailto:erik+lists@cederstrand.dk>> wrote:
Den 21/01/2014 kl. 08.58 skrev Johannes Schneider
> maybe I I formulated it not precise enough.
> I need to check if some instance of a model comes from some
database query (e.g. via get(..)) or is instanciated 'by hand'. and
not jet written to the database.
> In this case I don't need to know, if there is an related object
(e.g. with the same pk or same values for its attributes) in the
database.
Okay. Sounds like your best bet is to either roll your own, or use
the undocumented API as suggested by Tom Evans.
Either way, make sure to write tests so this doesn’t break when you
upgrade Django.
Erik
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Erik Cederstrand
Jan 21, 2014, 8:06:10 AM1/21/14
to Django Users
Den 21/01/2014 kl. 10.55 skrev Johannes Schneider <johannes....@galileo-press.de>:
> yes, but the problem is that we manually give them a pk.
>
> Maye it's the best to reduce the question to the following:
> Which (undocumented) field is better for this case?
> * instance._state.db (checking if this is None)
> * instance_state.adding (checking for True)
Pick whichever works for you. But make sure to write tests for this, so it still works when you upgrade Django.
> yes, but the problem is that we manually give them a pk.
>
> Maye it's the best to reduce the question to the following:
> Which (undocumented) field is better for this case?
> * instance._state.db (checking if this is None)
> * instance_state.adding (checking for True)
Erik
Johannes Schneider
Jan 21, 2014, 8:29:46 AM1/21/14
to django...@googlegroups.com
thnx guys for all that input.
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--
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
http://www.galileo-press.de/
Gesch�ftsf�hrer: Tomas Wehren, Ralf Kaulisch, Rainer Kaltenecker
Johannes Schneider
Webentwicklung
johannes....@galileo-press.de
Tel.: +49.228.42150.xxx
Galileo Press GmbH
Rheinwerkallee 4 - 53227 Bonn - Germany
Tel.: +49.228.42.150.0 (Zentrale) .77 (Fax)
http://www.galileo-press.de/
HRB 8363 Amtsgericht Bonn
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