Dynamically adding a field to a model - via contribute_to_class

Hi,

I don't know if crossposting with stackoverflow is allowed here but essentially my problem is explained in https://stackoverflow.com/questions/46162104/django-dynamic-model-fields-and-migrations.

What I want to create is a custom ModelField of type DecimalField, that dynamically adds another ModelField (CharField) to the source model. As far as I can tell, this is nicely explained in https://blog.elsdoerfer.name/2008/01/08/fuzzydates-or-one-django-model-field-multiple-database-columns/.

Let's assume I start with a fresh project and app and add this code to models.py:

from django.db import models
from django.db.models import signals


_currency_field_name = lambda name: '{}_extension'.format(name)


class PriceField(models.DecimalField):

    def contribute_to_class(self, cls, name):
        # add the extra currency field (CharField) to the class
        if not cls._meta.abstract:
            currency_field = models.CharField(
                max_length=3, 
                editable=False,
                null=True, 
                blank=True
            )
            cls.add_to_class(_currency_field_name(name), currency_field)
        # add the original price field (DecimalField) to the class
        super().contribute_to_class(cls, name)

        # TODO: set the descriptor
        # setattr(cls, self.name, FooDescriptor(self))


class FooModel(models.Model):

    price = PriceField('agrhhhhh', decimal_places=3, max_digits=10, blank=True, null=True)

If I then call ./manage.py makemigrations the following migration file for the app is created:

# Generated by Django 1.11.4 on 2017-09-11 18:02
from __future__ import unicode_literals

from django.db import migrations, models
import testing.models


class Migration(migrations.Migration):

    initial = True

    dependencies = [
    ]

    operations = [
        migrations.CreateModel(
            name='FooModel',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('price', testing.models.PriceField(blank=True, decimal_places=3, max_digits=10, null=True, verbose_name='agrhhhhh')),
                ('price_extension', models.CharField(blank=True, editable=False, max_length=3, null=True)),
            ],
        ),
    ]

All good, as far as I can tell. The problem comes up if I then call ./manage.py migrate which errors with the following exception:

./manage.py migrate testing
Operations to perform:
  Apply all migrations: testing
Running migrations:
  Applying testing.0001_initial...Traceback (most recent call last):
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 63, in execute
    return self.cursor.execute(sql)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 326, in execute
    return Database.Cursor.execute(self, query)
sqlite3.OperationalError: duplicate column name: price_extension

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
    utility.execute()
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/__init__.py", line 355, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
    fake_initial=fake_initial,
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/executor.py", line 115, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/migration.py", line 129, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/migrations/operations/models.py", line 97, in database_forwards
    schema_editor.create_model(model)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 303, in create_model
    self.execute(sql, params or None)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 120, in execute
    cursor.execute(sql, params)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 80, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/cachalot/monkey_patch.py", line 113, in inner
    out = original(cursor, sql, *args, **kwargs)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/utils.py", line 63, in execute
    return self.cursor.execute(sql)
  File "/usr/local/var/pyenv/versions/stockmanagement-3.6.2/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 326, in execute
    return Database.Cursor.execute(self, query)
django.db.utils.OperationalError: duplicate column name: price_extension

As said, I have a fresh project with no exisiting DB and tables so far. Why is it complaining that there allready exista a column named "price_extension"?. The migration file only contains one field called "price_extension"?

A sample project can be cloned from https://github.com/hetsch/django_testing. 

Thank you a lot for your help!

--
You received this message because you are subscribed to the Google Groups "Django users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscribe@googlegroups.com.
To post to this group, send email to django-users@googlegroups.com.
Visit this group at https://groups.google.com/group/django-users.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/9ca1469c-abcb-4b30-a383-f4654f1705b2%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.