Update a single column of a row in a Model

[Ken]

Folks

This is a bit of a newbie question on Model updates. I've been
happily using Django to give my users a read-only view of the database
for nearly a year, but now I'd like my users to be able to update a
row in one of my tables.

We're told in the Model documentation that "Django knows best" when it
comes to talking to the database.

However, I want my users to ONLY update ONE column in one row in my
database, but Django seems to insist on updating all the columns. (A
related problem is that I'd also like the database user only to be
allowed to update a subset of columns of my table - mostly because I'm
paranoid.)

If you don't specify the column value, and the column is nullable, the
unspecified columns get set to NULL instead of left alone.

Here's an example...

select * from tcs_detection_lists;
+----+-----------+---------------------+
| id | name | description |
+----+-----------+---------------------+
| 0 | garbage | Bad Candidates |
| 1 | confirmed | Confirmed SNe |
| 2 | good | Good Candidates |
| 3 | possible | Possible Candidates |
| 4 | pending | Not Yet Eyeballed |
+----+-----------+---------------------+

I want to update the word 'garbage' to 'bad'.

Here's my simplistic Django code...

dl = views.TcsDetectionLists(id=0, name ='bad')
dl.save()

select * from tcs_detection_lists;
+----+-----------+---------------------+
| id | name | description |
+----+-----------+---------------------+
| 0 | bad | |
| 1 | confirmed | Confirmed SNe |
| 2 | good | Good Candidates |
| 3 | possible | Possible Candidates |
| 4 | pending | Not Yet Eyeballed |
+----+-----------+---------------------+

I only wanted the user to change one column, but Django has assumed
None for the rest of the columns and therefore deleted my
'description' field.

The update that I want is:

update tcs_detection_lists
set name = 'bad'
where id = 0;

Note that some of my tables have many columns (>50) with a lot of
floating point data and I certainly don't want my users inadvertently
deleting data.

Do I need to specify something in my Model columns to get Django to
behave the way I want it to...? I really don't want to have to resort
to a bit of custom SQL...

Thanks in advance,

Ken

[Shawn Milochik]

Use a forms.ModelForm and exclude all the fields except one, or use a
forms.Form with one field and use its value in a queryset .update()
call.

Shawn

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[Ken]

Thanks

Unfortunately I'm very new to Forms/ModelForms, and I'm having a lot
of difficulty understanding the Django examples.

All I want is to allow my users to make one change to one column. I
don't want them to see the ID, just a text box and a submit button.
Hitting Submit should take them back to exactly the same page, with
the text field pre-populated to what they entered.

The database update should only change one column of the relevant row
in the database table.

This would take 30 seconds to do in PHP.

Can anyone point me to some simple examples (other than the Django
site).

Cheers,

Ken

On 3 Mar, 00:31, Shawn Milochik <sh...@milochik.com> wrote:
> Use a forms.ModelForm and exclude all the fields except one, or use a  
> forms.Form with one field and use its value in a queryset .update()  
> call.
>
> Shawn
>
> Sent from my iPhone
>

[Shawn Milochik]

Here's a simple example. It could be improved, but it's meant to be very simple.

#Make a simple form.

class AgeForm(forms.Form):
age = forms.IntegerField()

#When the user submits the form:

age_form = AgeForm(request.POST)

if age_form.is_valid()

#get the pk however you need to
person = Person.objects.get(pk = 123)
person.age = age_form.cleaned_data['age']
person.save()

[Ken]

Thanks Shawn

My problem is that Person.save() will do an update of all my columns.
Even though they are all identical, apart from the changed value, this
will violate my minimum privileges requirement of only allowing the
application access to the columns that it is allowed to change - hence
your original suggestion of using ModelForms (and only specified
fields).

Maybe if I try something like this...

#Use a ModelForm for Person..
class PersonForm(forms.ModelForm):
model=Person
fields = ('age')

#When the user submits the form:

person = Person.objects.get(pk = 123)

person_form = PersonForm(request.POST, instance = person)
if person_form.is_valid()
person_form.save()

Would that work...? I'll attempt to hack an example in my code and
tell you what happens...

Thanks for your time...

Ken

[Shawn Milochik]

1. Doing Person.save() will NOT update every field in your model.

2. Your snippet uses a ModelForm. Mine used a Form. There's a huge difference.
If you use a ModelForm you're going to have to exclude all the fields you don't want.

3. If you use a ModelForm and instantiate it with request.POST, don't send an argument
for 'instance' as well. One or the other.

Look at my example again. It does exactly what you want.
It seems that all your problems are due to not understanding the difference between a Form and a ModelForm.

Read the Django book ("The Definitive Guide").

Shawn

[Ken]

Thanks for your example, but whilst you're correct about Person.id not
getting updated, all the other columns do get changed (even if it is
to the same value).

Here's my code...

class TcsDetectionListsForm(forms.Form):
name = forms.CharField()

def candidateWithForm(request, tcs_transient_objects_id):
detectionListRow = TcsDetectionLists.objects.get(pk=0)

if request.method == 'POST':
form = TcsDetectionListsForm(request.POST)
if form.is_valid(): # All validation rules pass
detectionListRow.name = form.cleaned_data['name']
detectionListRow.save()
else:
form = TcsDetectionListsForm(initial={'name':
detectionListRow.name })

Here's what happened in the database (from the DB log):

223592 Query UPDATE `tcs_detection_lists` SET `name` = 'rubbish',
`description` = 'Bad Candidates' WHERE `tcs_detection_lists`.`id` = 0

We shouldn't be updating the 'description' column. If my security
settings were in place, this query would fail, because my DB user only
has update access to the 'name' column.

Ken

[Karen Tracey]

On Thu, Mar 4, 2010 at 2:24 PM, Ken <ken.w...@gmail.com> wrote:

Thanks for your example, but whilst you're correct about Person.id not
getting updated, all the other columns do get changed (even if it is
to the same value).

Here's my code...

class TcsDetectionListsForm(forms.Form):
   name = forms.CharField()

def candidateWithForm(request, tcs_transient_objects_id):
   detectionListRow = TcsDetectionLists.objects.get(pk=0)
 

   if request.method == 'POST':
       form = TcsDetectionListsForm(request.POST)
       if form.is_valid(): # All validation rules pass
           detectionListRow.name = form.cleaned_data['name']
           detectionListRow.save()
   else:
       form = TcsDetectionListsForm(initial={'name':
detectionListRow.name })

Here's what happened in the database (from the DB log):

223592 Query    UPDATE `tcs_detection_lists` SET `name` = 'rubbish',
`description` = 'Bad Candidates' WHERE `tcs_detection_lists`.`id` = 0

We shouldn't be updating the 'description' column.  If my security
settings were in place, this query would fail, because my DB user only
has update access to the 'name' column.

TcsDetectionLists.objects.filter(pk=0).update('name'=form.cleaned_data['name'])

will produce an SQL UPDATE that only touches the name column.

Karen
 

[Ken]

Many thanks Karen. This worked perfectly (though the 'name' attribute
is just name without the quotes).

Cheers,

Ken

On 5 Mar, 01:42, Karen Tracey <kmtra...@gmail.com> wrote:

> TcsDetectionLists.objects.filter(pk=0).update('name'=form.cleaned_data['nam e'])