CharField with fixed length, how?

Question

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? 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…

Accepted Answer

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)
```

CharField with fixed length, how?

Solution

Problem