Django ORM: default value from sql query

[Marcin Szamotulski]

Dear Django users,

I am would like to set a default value on a model's field as a result of
a simple query. Something that in SQLAlchemy is done like that:
http://docs.sqlalchemy.org/en/rel_0_7/core/schema.html#sql-expressions

Specifically I want to count some objects in the database and set this
count as the default value. I know that this can be done in two steps
first find the value then set it, but I would like to have it done
automatically.

Best regards,
Marcin

[Simon Charette]

You can achieve this by setting a callable default value [1] on you field:

class MyModel(models.Model):
    count = models.PositiveIntegerField(default=lambda:MyModel.objects.count())

[1] https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.Field.default

[Marcin Szamotulski]

I should be more specific: I cannot because the query depends on the
object instance:

class MyModel(models.Models):

fkey = models.ForeignKey(..., null=False)

count = models.PosotiveIntegerField(default=lambda:MyModel.objects.filter(fkey_id= _?_ ).count())


where _?_ should be the models's instance fkey attribute.

On 17:05 Wed 21 Aug , Simon Charette wrote:
> You can achieve this by setting a callable default value [1] on you field:
>
> class MyModel(models.Model):
> count =
> models.PositiveIntegerField(default=lambda:MyModel.objects.count())
>
> [1]
> https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.Field.default

> Le mercredi 21 ao�t 2013 18:49:48 UTC-4, Marcin Szamotulski a �crit :

> >
> > Dear Django users,
> >
> > I am would like to set a default value on a model's field as a result of
> > a simple query. Something that in SQLAlchemy is done like that:
> > http://docs.sqlalchemy.org/en/rel_0_7/core/schema.html#sql-expressions
> >
> > Specifically I want to count some objects in the database and set this
> > count as the default value. I know that this can be done in two steps
> > first find the value then set it, but I would like to have it done
> > automatically.
> >
> > Best regards,
> > Marcin
> >
>

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[Jason Arnst-Goodrich]

I don't think that will work because the default value is unrelated to a specific instance of a model and you want it to depend on the instance if I'm not mistaken. Once the instance exists, we're past the point where a default value will be used.

I think you'll have to handle this in a view.

On Wednesday, August 21, 2013 8:07:51 PM UTC-7, Marcin Szamotulski wrote:

I should be more specific:  I cannot because the query depends on the
object instance:

class MyModel(models.Models):

    fkey = models.ForeignKey(..., null=False)

    count = models.PosotiveIntegerField(default=lambda:MyModel.objects.filter(fkey_id= _?_ ).count())


where _?_ should be the models's instance fkey attribute.


On 17:05 Wed 21 Aug     , Simon Charette wrote:
> You can achieve this by setting a callable default value [1] on you field:
>
> class MyModel(models.Model):
>     count =
> models.PositiveIntegerField(default=lambda:MyModel.objects.count())
>
> [1]
> https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.Field.default

> Le mercredi 21 ao�t 2013 18:49:48 UTC-4, Marcin Szamotulski a �crit :