Set two fields to the same value

[Jaroslav Dobrek]

Hello,

how can I design a model such that two fields are set to the same value 
when a new object of the type defined by the model is created?

Example:

class Test(models.Model):
start_time = models.DateTimeField()
end_time = models.DateTimeField()
time = start_time

Users may create Test objects in order to run their own tests. A test 
always starts at some date time and it always ends at some date time. Each 
test has a time which is increased until it equals the end time. When a 
user creates a new test (and before he uses it) the field time should have 
the same value as the field start_time.

The code above will produce a database error.

Jaroslav

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Re: calculate default values for fields based on the content of other fields

[Jaroslav Dobrek]

On 5 Apr., 10:40, Xavier Ordoquy  wrote:
> Hi,
>
> I'm not sure I clearly understood your issue.
> Do you want some choice fields to limit their choices according to another 
> choice field value ?

No. I want to use prepopulated editable char fields. The content of
those fields should be calculated using the content of another char
field.



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Re: calculate default values for fields based on the content of other fields

[Jaroslav Dobrek]

> However, I don't think you need to do this at all. I would say you
> would only populate the other fields if they are all empty, and the
> base is not empty. If the other fields are not empty, either it has
> already been pre-populated, or it has been filled in manually, and you
> should not change it. Seems pretty straightforward!

I think this is not an option: In the majority of cases, correcting a
form will mean to delete it.
Example: The program might generate the comparative form "magicaler"
and the superlative form
"magicalest" for the English adjective "magical". The administrator's
job is to delete these
forms. Of course, I might require every field to have a content and
use some special symbol, such
as '-' to denote that a field is empty. But I think this would
unnecessarily complicate the coders'
work. The fewer rules and conventions the coder needs to know, the
better. And it wouldn't help
either, because sometimes - for example in the case of English
adjectives of a certain length -
the program would assign this symbol or the empty string itself.

> save() happens before the data is saved to the database (well, its the
> last things that happens in Model.save()). So you can pull the
> original data from the database, and compare it to the one you are
> being asked to save.

I don't quite understand you. Where exactly is the data I am "being
asked to save"? The following
function might be an approximation to what you suggest:

def save(self, *args,
**kwargs):
#1
tmp =
self.attr_pos_str_masc_nom_sing
#2
if tmp !=
self.default_attr_pos_str_masc_nom_sing()
#3
 
pass
#4
 
else:
#5
self.attr_pos_str_masc_nom_sing =
self.default_attr_pos_str_masc_nom_sing() #6
super(GermanAdjective, self).save(*args,
**kwargs)   #7

(I hope this will keep its indentation.)

In line 2, I look up the value of the field
attr_pos_str_masc_nom_sing. In line 3, I check
if the current value of this field differs from the default value for
this field. If so, I assume that
the field has been edited and do nothing. Yet, when an adjective is
created, this field is empty,
so the test is positive and nothing is done.

Jaroslav

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Re: calculate default values for fields based on the content of other fields

[Jaroslav Dobrek]

> You could override Model.save() on the model you wish to calculate
> fields for. This could populate the fields if they are not already
> populated and the instance has a base adjective.
>

I had thought of this possibilty, too. I would prefer it, because it
is the simplest one.
But I don't understand how to find out in the save() function if the
admin has modified the field.
If I don't know this, the field will get the default value every time
an administrator saves the word.

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calculate default values for fields based on the content of other fields

[Jaroslav Dobrek]

Hello,

is there a way to calculate default values for certain fields based
on the content of other fields?

I am implementing a dictionary. Admins are supposed to add words of
certain languages and parts-of-speech. Some words of some
part-of-speech and language are, on one hand, morphologically very
regular, but, one the other hand, come with a huge number of
morphological
forms. This is true for German adjectives: They usually have 183
morphological forms (if form-meaning pairs are counted). On the other
hand, there are very few irregular adjectives. The most user-friendly
way for admins to code German adjectives would be this: The admin adds
a new adjective and types in the adjective's base form:

base form: |gut|

Then he clicks some button or saves the adjective or, ideally, does
nothing, and the program fills in the 183 fields below, using the base
form and
assuming that the adjective is regular:

attributive positive strong masculine nominative singular: |guter|
attributive positive strong masculine genitive singular:   |guten|
attributive positive strong masculine dative singular: |gutem|
attributive positive strong masculine accusative singular: |guten|
...
attributive comparative strong masculine nominative singular: |
guterer|
...

The admin now has the possibility to correct the wrong forms before
saving the adjective. This way, in the vast majority of cases all the
coding is done completely
automatically. And for the very few irregular adjectives, the admin
only has to modify those
forms that have been guessed wrongly.

Any ideas how to realize this?

Jaroslav

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field of tuples

[Jaroslav Dobrek]

Hello,

how can I create a field that contains tuples of entities? An example:
A model "Candidate" (someone who applies for a job) might have a field
"languages" which is a list of tuples consisting of a language and a
proficiency level.

Jaroslav

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field name hiding workaround

[Jaroslav Dobrek]

Hello,

I have got a class Lexeme which  I would like to define as follows:

class Lexeme(models.Model):

class Meta:
unique_together = ((u"entity", u"language"),)

language = models.ForeignKey(Language)

# ...

I would like to have different types of lexemes, each of which has
another field type as "entity". Like so:


class CommodityLexeme(Lexeme):

class Meta:
verbose_name_plural = "Commodities"

entity = models.ForeignKey(Commodity)


class CountryLexeme(Lexeme):

class Meta:
verbose_name_plural = "Countries"

entity = models.ForeignKey(Country)


I know this doesn't work, because field name hiding is not permitted.
(See 
https://docs.djangoproject.com/en/1.1/topics/db/models/#field-name-hiding-is-not-permitted).

But what would be the most elegant workaround? I suppose someone has
had a similar problem before.

Jaroslav

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Re: document admin interface

[Jaroslav Dobrek]

On 16 Jan., 16:36, Sébastien Billion 
wrote:
> Hi,
>
> Why don't use help_text argument in your models or formAdmin?

Thank you. That was exactly what I was looking for.

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document admin interface

[Jaroslav Dobrek]

Hello,

how can I document the admin interface. I do not mean the stuff that
can be achieved with the admin documentation generator. I would like
to add documentation to those pages that admins see when they create,
delete or modify an object. I.e. I want to add explanations about the
object being manipulated.

Jaroslav

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sorting of foreign key elements

[Jaroslav Dobrek]

Hello,

is there a way to change the order in which foreign key elements are
displayed in the admin interface?

I have got a model Currency that has a field currency_id:

currency_id = models.ForeignKey(CurrencyID)

("Currencies" are language-dependent ways of referring to currencies,
"CurrencyIDs" are language-independent objects.)

Although my CurrencyID objects are sorted alphabetically (Afghan
Afghani, Albanian Lek, Algerian Dinar, ...) they appear in reversed
order in the currency_id slot in the admin interface of Currency
objects (Zimbabwe Dollar, Zambian Kwacha, Yemeni Rial ...). Can I
change this? If so, how?

Jaroslav

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Re: clean function for field that may be blank

[Jaroslav Dobrek]

> https://docs.djangoproject.com/en/1.3/ref/models/fields/#blankhttps://docs.djangoproject.com/en/1.3/ref/models/fields/#null
>
> The confusion comes about because charfields do not require null=True,
> as empty or missing strings are stored as the empty string rather than
> as a null string.
>

The code works with null=True.

Thanks for pointing this out.

Jaroslav

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Re: clean function for field that may be blank

[Jaroslav Dobrek]

I forgot to say:

(1) I deleted the database completely and created it again.
(2) I ran manage.py syncdb

However, with this code

 def clean_date_of_birth(self):
 date_of_birth = self.cleaned_data.get('date_of_birth')
 if date_of_birth:
 if date_of_birth.year < 1890:
 raise forms.ValidationError(u"It is very
improbable...")
 return date_of_birth

I still get the database problem.

Jaroslav

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Re: clean function for field that may be blank

[Jaroslav Dobrek]

>      def clean_date_of_birth(self):
>          date_of_birth = self.cleaned_data.get('date_of_birth')
>          if date_of_birth:
>              if date_of_birth.year < 1890:
>                  raise forms.ValidationError(u"It is very improbable...")
>          return date_of_birth
>

I had already tried this. It will produce a database error: According
to the postgresql server date_of_birth must not be null:

FEHLER:  NULL-Wert in Spalte »date_of_birth« verletzt Not-Null-
Constraint

(Unfortunately, our postgresql server produces German error messages
and our system administrator will  not change this. I don't know the
exact wording of this error message in English.)



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clean function for field that may be blank

[Jaroslav Dobrek]

Hello,

I have got a class Person:

class Person(models.Model):
given_names = models.CharField(max_length=200)
family_names = models.CharField(max_length=200)
date_of_birth = models.DateField(blank=True)

and a class PersonForm:

class PersonForm(forms.ModelForm):

def clean_date_of_birth(self):
try:
date_of_birth = self.cleaned_data['date_of_birth']
if date_of_birth.year < 1890:
raise forms.ValidationError(u"It is very improbable
for a person to be this old.")
return date_of_birth
except KeyError:
pass

The code works as expected if I leave out "blank=True" in the field
date_of_birth of the class Person. But it doesn't work with
blank=True.

How does clean_date_of_birth have to look like, if date_of_birth is
optional?

Jaroslav

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Re: Set of choices of which several can be chosen without using a ManyToManyField

[Jaroslav Dobrek]

On 1 Nov., 18:54, Mark Furbee  wrote:
> As alluded to previously, the most "straightforward way to use a set of
> choices of which several can be chosen" IS to use a ManyToManyField. The
> syntax is slightly different, but ManyToManyFields are really easy to use
> with Django. Do not reinvent the wheel in this case.

I am not against ManyToManyFields. Yet, in certain cases (which are
actually more complicated than the (invented) Candidate/language
example), I would like to not create certain data as objects that can
be seen, modified, created and deleted by administrators.
Administrators should only see those data which they are supposed to
manipulate. That is why I wanted to have the possibility to "hardwire"
certain data in my source code in the same way as data is hardwired in
the source code with the "choices=" solution that can be used with
CharField.

I now use ManyToManyFields and hide certain data from administrators
by simply not importing them into admin.py. What I don't like about
this solution is that this data still is in the database and not in my
source code. In my view, it should be part of the source code, because
it is part of the program logic: Any instance of my program is
supposed to use these data, independently of any other set of data it
might use. Much as any instance of a program that uses the notion of
gender should have the genders "male" and "female". Administrators
should not have to or be able to manipulate gender objects.

Thanks to all who participated in this thread.

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Re: Set of choices of which several can be chosen without using a ManyToManyField

[Jaroslav Dobrek]

> You are confusing model fields with form fields. MultipleChoiceField
> is a form field, not a model field.

I wasn't aware of the existence of MultipleChoiceFields. The idea of
the above code was to express that I wanted to use this code

class Candidate(models.Model):

programming_languages = models.CharField(max_length=50, choices=(

(u'Python)', u'Python'),
(u'C++', u'C++'),
(u'Java', u'Java'),
# ...
), blank=True)

with the only exception that, in the admin interface, several choices
are possible when one creates a new candidate object. I.e. I want
admins to be able to create a candidate that knows, say Python *and* C+
+ by choosing both of these languages during the creation of the
object. I used the string "MultipleChoiceField" as a dummy for
whatever should be used instead.

Jaroslav






> If you want a field that will be represented by a MultipleChoiceField
> in model, you simply need to define 'choices' on a field class.
>
> https://docs.djangoproject.com/en/1.3/ref/models/fields/#choices
>
> Cheers
>
> Tom

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Set of choices of which several can be chosen without using a ManyToManyField

[Jaroslav Dobrek]

Hello,

what is the most straightforward way to use a set of choices of which
several can be chosen without using a ManyToManyField?

Using a ManyToManyField would make my program unnecessarily
complicated.

Example:

class Candidate(models.Model):

programming_languages = models.MultipleChoiceField(max_length=50,
choices=( # replace MultipleChoiceField with existing possibility

(u'Python)', u'Python'),
(u'C++', u'C++'),
(u'Java', u'Java'),
# ...
), blank=True)

Jaroslav

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Re: create object instance with ManyToManyField

[Jaroslav Dobrek]

Hello Leonardo,

thanks your your answer.

> n = Company(name=my_name, country=my_country, isin=my_isin)
> n.save()
> n.indices.add(my_indices)

This causes that the company object has all indices in my_indices
(such as Dow Jones S 100, Dax, ...) as *choices*.  I.e. someone who
manipulates these objects via the admin interface is now able to
choose one or several of them and save the object again. What I want
to do is choose one or several of the indices in my program and then
save the object.

Jaroslav

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create object instance with ManyToManyField

[Jaroslav Dobrek]

Hello,

how can I create an object instance with a ManyToManyField. Everything
works fine via the admin interface. But I don't understand how to do
it in Python.

Example: I have this model:

class Company(models.Model):

name = models.CharField(max_length=200, unique=True)
country = models.ForeignKey(Country)
isin = models.CharField(max_length=12, blank=True)
indices = models.ManyToManyField(Index, blank=True)

def the_indices(self):
ind = []
for index in self.indices.all():
ind.append(index.name)
return ', '.join(ind)
the_indices.short_description = u'Indices'

def __unicode__(self):
return self.name

This will work:

n = Company(name=my_name, country=my_country, isin=my_isin)
n.save()

This will not work:

n = Company(name=my_name, country=my_country, isin=my_isin,
indices=my_indices)
n.save()

Although I make sure that my_indices is a list of existing index
objects.

What do I have to do?

Jaroslav




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Re: store superuser permamently

[Jaroslav Dobrek]

On 3 Feb., 15:19, Ivo Brodien  wrote:
> > What do I have to do to avoid having to define a superuser each time?
>
> you can dump the data of the auth app and than load it again.
>
> s.th. like this:
>
> ./manage.py dumpdata --indent=4 auth  > fixtures/auth.json
> ./manage.py dumpdata --indent=4 sessions >  fixtures/sessions.json
>
> you can do a manage.py syncdb —noinput so you will not be asked, if you want 
> a superuser or not.
>
> If you dump the sessions too you will stay logged in.

Thank you. That is exactly what I wanted.

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store superuser permamently

[Jaroslav Dobrek]

Hello,

I often use

manage.py dumpdata --indent 2 > initial_data.json

and then manipulate initial_data.json.

When I remove my database file and run manage.py syncdb almost all
data from inital_data.json are installed. (Many errors only go away
when the database file is removed.) But manage.py asks me each time:

You just installed Django's auth system, which means you don't have
any superusers defined.
Would you like to create one now? (yes/no):

And then I have to type "yes", type the superuser's name, email
adress ...

When you say "no", the data is correctly installed, but you can't log
in to the admin interface. (I thought a system without a superuser
would not be password protected. What point is there in being able to
not define a superuser, if the system cannot be used without one?)

What do I have to do to avoid having to define a superuser each time?

Jaroslav

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Re: abstract base class and overwriting save()

[Jaroslav Dobrek]

> The whole point of subclassing is that subclasses inherit models from the
> superclass. That's how the model save() method is definted in the first
> place, if you don't override it. What exactly are you having problems with?

It seems that my problems were due to another mistake of mine. I was
thinking in a too complicated way. You are right: nothing has to be
done, just inherit. Thank you.

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abstract base class and overwriting save()

[Jaroslav Dobrek]

Hi,

if I have got several models that have identical code, except for
their save method, how can I use an abstract base class for them? Will
I have to overwrite save() for each model? Or is there a generic way
to define save(), i.e. a way that I have to use only once -- in the
abstract base class?

class BMW(models.Model):
   ...
  save def save(self, *args, **kwargs):
do_something()
super(BMW, self).save(*args, **kwargs)

class Fiat(models.Model):
   ...
  save def save(self, *args, **kwargs):
do_something()
super(Fiat, self).save(*args, **kwargs)

Jaroslav

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Re: forbid clones

[Jaroslav Dobrek]

On 26 Jan., 13:29, Chris Matthews  wrote:
> Have a look at unique_together:
> Django | Model Meta options | Django 
> documentationhttp://docs.djangoproject.com/en/dev/ref/models/options/
>
> Jump to unique_togetherý: Options.unique_together¶. Sets of field names that, 
> ... For convenience, unique_together can be a single list when dealing ...

Great, thank you. That is exactly what I was looking for.

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Re: forbid clones

[Jaroslav Dobrek]

>
> http://docs.djangoproject.com/en/1.2/ref/models/fields/#unique

Although this does help, it leaves one question open:

How can we forbid only such pairs of objects that have the same value
in all of their attributes.

Example:

This should be allowed:

car1: manufacturer = "foo", name = "bar"
car2: manufacturer = "foo", name = "baz"

This should not be allowed:

car1: manufacturer = "foo", name = "bar"
car2: manufacturer = "foo", name = "bar"


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Re: forbid clones

[Jaroslav Dobrek]

> http://docs.djangoproject.com/en/1.2/ref/models/fields/#unique
>

Wow, thanks. In this group answers almost come before one is able to
submit the posting.

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forbid clones

[Jaroslav Dobrek]

Hi,

how can I forbid that clones of a certain type are stored by an admin?

Example: Suppose there are cars in the database and cars have as only
attribute a name. How can I prevent that two cars that have the same
name are stored?

Jaroslav

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Re: method of method

[Jaroslav Dobrek]

Thanks to all of you.

Now I understand this better.

Jaroslav

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Re: method of method

[Jaroslav Dobrek]

On 25 Jan., 12:36, Euan Goddard  wrote:
> I agree with the previous poster - the title is misleading as the word
> "method" is incorrect in both places.

O.k. sorry. Should have been "attribute of attribute", right?

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Re: method of method

[Jaroslav Dobrek]

>> What happens ?

I get this error message when I run manage.py syncdb:

AttributeError: 'ForeignKey' object has no attribute 'producer'

>> And what did you expect to happen ?

I want the 'country_of_origin' of cars to be the string that is stored
in their producer's 'country_of_origin':

class CarProducer(models.Model):
  country_of_origin = models.CharField(max_length=200)


class Car(models.Model):
    producer = models.ForeignKey(CarProducer)
    country_of_origin = producer.country_of_origin

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method of method

[Jaroslav Dobrek]

Hi,

can I use methods of methods in Django?

Like so:

class Car(models.Model):
producer = models.ForeignKey(CarProducer)
country_of_origin = producer.country_of_origin

This doesn't seem to work. Is there a special syntax for this?

Jaroslav

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