django models - conditionally set blank=True

Question

I am trying to build an app, where user can customize forms. The Following example contains classes for creating Fields (QuestionField, AnswerField) which is used by the admin and the BoolAnswer which is filled by the user: This Way an admin can create a Form with questions and possible answers.

According to django documentation, blank=True is related to evaluation. The problem is that it is set on the class level rather than on object level.

How can I set blank=True depending on the related model so that I do not have to reimplement an own validator? (see pseudo code in BoolAnswer)

My models.py:

class QuestionField(models.Model):
    question = models.TextField(max_length=200)
    models.ForeignKey(Sheet)


class BoolAnswerField(AnswerField):
    question = models.ForeignKey(models.Model)
    if_true_field = models.TextField(max_length=100, null=True)


class BoolAnswer(models.Model):
    bool_answer_field = models.ForeignKey(BoolAnswerField)
    result = models.BooleanField()
    if_true = models.TextField(max_length=100, null=True,

                               blank=True if self.bool_answer_field.if_true_field)

** Short explanation **: If the Answer to a BoolAnswerField question is True, if_true field should explain, why

Answer 1

Don't hate me, but validation is the way to go, see here

class BoolAnswer(models.Model):
    bool_answer_field = models.ForeignKey(BoolAnswerField)
    result = models.BooleanField()
    if_true = models.TextField(max_length=100, null=True, blank=True)

    def clean(self)
        if self.bool_answer_field.if_true_field and not self.if_true:
            raise ValidationError('BAF is True without a reason')

In case you want your error message to be displayed next to the field, not at the beginning of the form, you've got to pass a dict to ValidationError, like:

from django.utils.translation import gettext_lazy as _
...
raise ValidationError({
    'field_name': _('This field is required.')})