Why can't "directly" change array elements of model objects in Django shell?

[Chris Seberino]

I've noticed that this doesn't often work in Django shell...

x = MyModel.objects.all()
x[3].some_field = "new value"
x[3].save()

Rather, I must often do this...

temp = x[3]
temp.some_field = "new value"
temp.save()

*Sometimes* the first works and I don't know why.

Please advise.

Chris

[Jacob Kaplan-Moss]

This is because `MyModel.objects.all()` isn't a list; it's a QuerySet.
That is, `MyModel.objects.all()` *doesn't* hit the database until you
say `x[3]`, at which point Django performs a query with a LIMIT and
OFFSET to just select that 3rd item. The entire list is never fetched.

See https://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated
for more details.

Jacob

[Javier Guerra Giraldez]

On Fri, Oct 7, 2011 at 10:39 AM, Jacob Kaplan-Moss <ja...@jacobian.org> wrote:
>> *Sometimes* the first works and I don't know why.
>
> This is because `MyModel.objects.all()` isn't a list; it's a QuerySet.
> That is, `MyModel.objects.all()` *doesn't* hit the database until you
> say `x[3]`, at which point Django performs a query with a LIMIT and
> OFFSET to just select that 3rd item. The entire list is never fetched.

... and the QuerySet is free to override its own internal cache
anytime it wants. You know that `x[3]` will refer to the same
database record, but there's no guarantee that it will be the same
Python object.

--
Javier

[Chris Seberino]

On Oct 7, 10:39 am, Jacob Kaplan-Moss <ja...@jacobian.org> wrote:

> On Fri, Oct 7, 2011 at 10:30 AM, Chris Seberino <cseber...@gmail.com> wrote:
>
> > I've noticed that this doesn't often work in Django shell...
>
> > x = MyModel.objects.all()
> > x[3].some_field = "new value"
> > x[3].save()

> This is because `MyModel.objects.all()` isn't a list; it's a QuerySet.
> That is, `MyModel.objects.all()` *doesn't* hit the database until you
> say `x[3]`, at which point Django performs a query with a LIMIT and

OK. I understand. What is easiest way then to *force* my x[3] code
above to hit the database so that
my changes are permanent?

[Andre Terra]

The easiest way would be to slice the list exactly how you were doing with x[3]... It doesn't get much easier than that.

I guess if you really need to spare yourself three keystrokes, you can do it like this:

>>> x = YourModel.objects.all()[3]
>>> x.foo = 'bar'
>>> x.save()


Cheers,
AT

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[Tom Evans]

x = list(MyModel.objects.all())

x[3].some_field = "new value"
x[3].save()

Cheers

Tom

[fei]

Instead, you can use the get() function,

x = MyModel.objects.get(id=3)
x.some_field = "new value"
x.save()

The difference between all() and get() is that all() will do a lazy
evaluation and get() will hit the physical database.

Fei

On Oct 12, 11:57 pm, Tom Evans <tevans...@googlemail.com> wrote: