How can I impose these constraints on my model using django?

Question

This is my model:

class MyModel(models.Model): s1 = models.CharField(max_length=255) s2 = models.CharField(max_length=255)

I was expecting django not to allow me to have empty strings considered as valid, as I read in the documentation that both blank and null are by default false. Example:

python manage.py shell

>>> a = MyModel() >>> a.s1 '' >>> a.save() >>> a.id() 1

Why didn't it raise an exception that the string was blank? Also, is there no such thing as a min_length? I want to limit my fields to at least 3 chars.

Shawn Chin · Accepted Answer · 2011-06-06 17:45:24Z

blank is a validation-related attribute, which means it will only apply when data is entered via the django-admin site or a form derived from the model using ModelForm.

Example:

class MyForm(forms.ModelForm): class Meta: model = MyModel >>> f = MyForm({"s1":""}) >>> f.is_valid() False >>> f = MyForm({"s1":"hello world"}) >>> f.is_valid() True >>> a = f.save() >>> a <MyModel: MyModel object> >>> a.id 10

min_length

min_length does exists, but as an argument for forms.CharField not models.CharField.

class MyForm(forms.ModelForm): class Meta: model = MyModel s1 = forms.CharField(min_length=3, max_length=100) >>> f = MyForm({"s1":"12"}) >>> f.is_valid() False >>> f = MyForm({"s1":"1234"}) >>> f.is_valid() True

Raising exceptions on unspecified char fields (without forms)

By setting default=None, the charfield will default to NULL instead of "" which will raise an exception when the model is saved manually (unless null=True is specified).

class MyModel(models.Model): s1 = models.CharField(max_length=255, default=None) >>> m = MyModel() >>> m.save() Traceback (most recent call last): File "<console>", line 1, in ? <.... trimmed for brevity ...> File "/usr/lib/python2.3/site-packages/django/db/backends/sqlite3/base.py", line 193, in execute return Database.Cursor.execute(self, query, params) IntegrityError: test_mymodel.s1 may not be NULL

I've only tested this on an old-ish version of django so YMMV

Update

From 1.2 onwards, django models come with validation built-in which means you can trigger the validation functions by calling Model.full_clean(), without having to use ModelForm.

class MyModel(models.Model): s1 = models.CharField(max_length=100) def save(self, *args, **kwargs): self.full_clean() Super(MyModel, self).save(*args, **kwargs) >>> m = MyModel() >>> m.save() Traceback (most recent call last): File "<console>", line 1, in <module> File "/tmp/lsc/playground/zoo/models.py", line 42, in save self.full_clean() File "/usr/lib/pymodules/python2.6/django/db/models/base.py", line 881, in full_clean raise ValidationError(errors) ValidationError: {'s1': [u'This field cannot be blank.']}

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