class Field(models.Model):
    name = models.CharField(u'Название поля',max_length=100,)
    type = models.CharField(u'Тип поля',max_length=20, choices=TYPE_OF_FIELD_CHOICES)
    
class Dept(models.Model):
   ...
    fields=models.ManyToManyField(Field)
   
class Answer(models.Model):
    user = models.ForeignKey(UserAnswer)
    field = models.ForeignKey(Field)
    data = models.TextField()

from django import forms
from models import Dept, Field

class DeptForm(forms.ModelForm):

    class Meta:
        model = Dept

    def __init__(self, *args, **kwargs):
        super(DeptForm, self).__init__(*args, **kwargs)
        self.fields['fields'] = forms.ModelChoiceField(
            queryset=self.instance.fields.all(), #if you add instance=Dept, when you create this form
        )