Migration clashes with forms.py

Question

The command python manage.py makemigrations fails most of time due to the forms.py, in which new models or new fields are referenced at class definition level.

So I have to comment each such definitions for the migration to operate. It's a painfull task.

I don't understand why the migration process import the forms.py module. I think that importing models modules should be sufficient.

Is there a way to avoid those errors ?

Answer 1

I was having this same issue and found the specific problem. When the migrate command was being called, Django's system checks made their way into my forms.py and then would fail when they encountered a line of code that made a query against a table that the migration was supposed to create. I had a choicefield that instantiated the choices with a database query like this:

university = forms.ChoiceField(
    choices=[('', '')] + [(university.id, university.name) for university in University.objects.all()],
    widget=forms.Select(
        attrs={
            'class': 'form-control',
            'placeholder': 'University',
        }
    ),
    required=True
)

The solution was to remove the query from choices (leaving it just as [('', '')] and then populate the choices in the class's init method instead.

class UniversityForm(forms.Form):

    university = forms.ChoiceField(
        choices=[('', '')],
        widget=forms.Select(
            attrs={
                'class': 'form-control',
                'placeholder': 'University',
            }
        ),
        required=True
    )


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

    # Load choices here so db calls are not made during migrations.
    self.fields['university'].choices = [('', '')] + [(university.name, university.name) for university in University.objects.all()]

Answer 2

Thanks to @alasdair I understood my problem and found a workaround: I replace the original code in the views.py file

from MyApp import forms

with

import sys
if 'makemigrations' not in sys.argv and 'migrate' not in sys.argv:
    from MyApp import forms

It works fine in my case, but I suppose there is a better way to know if the current process is a migration or not. If so, please advise.