Create Custom Error Messages with Model Forms

Another easy way of doing this is just override it in init.

class AuthorForm(forms.ModelForm):
    class Meta:
        model = Author

    def __init__(self, *args, **kwargs):
        super(AuthorForm, self).__init__(*args, **kwargs)

        # add custom error messages
        self.fields['name'].error_messages.update({
            'required': 'Please let us know what to call you!',
        })

For simple cases, you can specify custom error messages

class AuthorForm(forms.ModelForm):
    first_name = forms.CharField(error_messages={'required': 'Please let us know what to call you!'})
    class Meta:
        model = Author

New in Django 1.6:

ModelForm accepts several new Meta options.

• Fields included in the localized_fields list will be localized (by setting localize on the form field).
• The labels, help_texts and error_messages options may be used to customize the default fields, see Overriding the default fields for details.

From that:

class AuthorForm(ModelForm):
    class Meta:
        model = Author
        fields = ('name', 'title', 'birth_date')
        labels = {
            'name': _('Writer'),
        }
        help_texts = {
            'name': _('Some useful help text.'),
        }
        error_messages = {
            'name': {
                'max_length': _("This writer's name is too long."),
            },
        }

Related: Django's ModelForm - where is the list of Meta options?