Django: CharField with fixed length, how?

Question

Hi everybody, I wold like to have in my model a CharField with fixed length. In other words I want that only a specified length is valid.

I tried to do something like

volumenumber = models.CharField('Volume Number', max_length=4, min_length=4)

but it gives me an error (it seems that I can use both max_length and min_length at the same time).

Is there another quick way?

Thanks

EDIT:

Following the suggestions of some people I will be a bit more specific:

My model is this:

class Volume(models.Model):
    vid = models.AutoField(primary_key=True)
    jid = models.ForeignKey(Journals, db_column='jid', null=True, verbose_name = "Journal")
    volumenumber = models.CharField('Volume Number')
    date_publication = models.CharField('Date of Publication', max_length=6, blank=True)
    class Meta:
        db_table = u'volume'
        verbose_name = "Volume"
        ordering = ['jid', 'volumenumber']
        unique_together = ('jid', 'volumenumber')
    def __unicode__(self):
        return (str(self.jid) + ' - ' + str(self.volumenumber))

What I want is that the volumenumber must be exactly 4 characters.

I.E. if someone insert '4b' django gives an error because it expects a string of 4 characters.

So I tried with

volumenumber = models.CharField('Volume Number', max_length=4, min_length=4)

but it gives me this error:

Validating models...
Unhandled exception in thread started by <function inner_run at 0x70feb0>
Traceback (most recent call last):
  File "/Library/Python/2.5/site-packages/django/core/management/commands/runserver.py", line 48, in inner_run
    self.validate(display_num_errors=True)
  File "/Library/Python/2.5/site-packages/django/core/management/base.py", line 249, in validate
    num_errors = get_validation_errors(s, app)
  File "/Library/Python/2.5/site-packages/django/core/management/validation.py", line 28, in get_validation_errors
    for (app_name, error) in get_app_errors().items():
  File "/Library/Python/2.5/site-packages/django/db/models/loading.py", line 131, in get_app_errors
    self._populate()
  File "/Library/Python/2.5/site-packages/django/db/models/loading.py", line 58, in _populate
    self.load_app(app_name, True)
  File "/Library/Python/2.5/site-packages/django/db/models/loading.py", line 74, in load_app
    models = import_module('.models', app_name)
  File "/Library/Python/2.5/site-packages/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/Users/Giovanni/src/djangoTestSite/../djangoTestSite/journaldb/models.py", line 120, in <module>
    class Volume(models.Model):
  File "/Users/Giovanni/src/djangoTestSite/../djangoTestSite/journaldb/models.py", line 123, in Volume
    volumenumber = models.CharField('Volume Number', max_length=4, min_length=4)
TypeError: __init__() got an unexpected keyword argument 'min_length'

That obviously doesn't appear if I use only "max_length" OR "min_length".

I read the documentation on the django web site and it seems that I'm right (I cannot use both together) so I'm asking if there is another way to solve the problem.

Thanks again

Answer 1

CharField database model field instances only have a max_length parameter, as indicated in the docs. This is probably because there is only a max character length contraint equivalent in SQL.

Form Field CharField objects, on the other hand, do have a min_length parameter. So you'd have to write a custom ModelForm for this specific model and override the default admin model form with the custom one.

Something like that:

# admin.py

from django import forms

...

class VolumeForm(forms.ModelForm):
    volumenumber = forms.CharField(max_length=4, min_length=4)

    class Meta:
        model = Volume


class VolumeAdmin(admin.ModelAdmin):
    form = VolumeForm

...

admin.site.register(Volume, VolumeAdmin)