Django migration error :you cannot alter to or fro

Question

I'm trying to modify a M2M field to a ForeignKey field. The command validate shows me no issues and when I run syncdb :

ValueError: Cannot alter field xxx into yyy they are not compatible types (you cannot alter to or from M2M fields, or add or remove through= on M2M fields)

So I can't make the migration.

class InstituteStaff(Person):
    user                 = models.OneToOneField(User, blank=True, null=True)
    investigation_area   = models.ManyToManyField(InvestigationArea, blank=True,)
    investigation_group  = models.ManyToManyField(InvestigationGroup, blank=True)
    council_group        = models.ForeignKey(CouncilGroup, null=True, blank=True)
    #profiles            = models.ManyToManyField(Profiles, null = True, blank = True)
    profiles             = models.ForeignKey(Profiles, null = True, blank = True)

This is my first Django project so any suggestions are welcome.

Answer 1

Potential workarounds:

• Create a new field with the ForeignKey relationship called profiles1 and DO NOT modify profiles. Make and run the migration. You might need a related_name parameter to prevent conflicts. Do a subsequent migration that drops the original field. Then do another migration that renames profiles1 back to profiles. Obviously, you won't have data in the new ForeignKey field.

• Write a custom migration: https://docs.djangoproject.com/en/1.7/ref/migration-operations/

You might want to use makemigration and migration rather than syncdb.

Does your InstituteStaff have data that you want to retain?

Answer 2

If you're still developing the application, and don't need to preserve your existing data, you can get around this issue by doing the following:

1. Delete and re-create the db.

2. go to your project/app/migrations folder

3. Delete everything in that folder with the exception of the init.py file. Make sure you also delete the pycache dir.

4. Run syncdb, makemigrations, and migrate.

Answer 3

I stumbled upon this and although I didn't care about my data much, I still didn't want to delete the whole DB. So I opened the migration file and changed the AlterField() command to a RemoveField() and an AddField() command that worked well. I lost my data on the specific field, but nothing else.

I.e.

migrations.AlterField(
    model_name='player',
    name='teams',
    field=models.ManyToManyField(related_name='players', through='players.TeamPlayer', to='players.Team'),
),

to

migrations.RemoveField(
    model_name='player',
    name='teams',
),
migrations.AddField(
    model_name='player',
    name='teams',
    field=models.ManyToManyField(related_name='players', through='players.TeamPlayer', to='players.Team'),
),

Answer 4

NO DATA LOSS EXAMPLE

---

I would say: If machine cannot do something for us, then let's help it!

Because the problem that OP put here can have multiple mutations, I will try to explain how to struggle with that kind of problem in a simple way.

Let's assume we have a model (in the app called users) like this:

from django.db import models


class Person(models.Model):
    name = models.CharField(max_length=128)

    def __str__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person)

    def __str__(self):
        return self.name

but after some while we need to add a date of a member join. So we want this:

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership') # <-- through model

    def __str__(self):
        return self.name

# and through Model itself
class Membership(models.Model):
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    date_joined = models.DateField()

Now, normally you will hit the same problem as OP wrote. To solve it, follow these steps:

• start from this point:

  from django.db import models
  
  
  class Person(models.Model):
      name = models.CharField(max_length=128)
  
      def __str__(self):
          return self.name
  
  class Group(models.Model):
      name = models.CharField(max_length=128)
      members = models.ManyToManyField(Person)
  
      def __str__(self):
          return self.name
  

• create through model and run python manage.py makemigrations (but don't put through property in the Group.members field yet):

  from django.db import models
  
  
  class Person(models.Model):
      name = models.CharField(max_length=128)
  
      def __str__(self):
          return self.name
  
  class Group(models.Model):
      name = models.CharField(max_length=128)
      members = models.ManyToManyField(Person) # <-- no through property yet!
  
      def __str__(self):
          return self.name
  
  class Membership(models.Model): # <--- through model
      person = models.ForeignKey(Person, on_delete=models.CASCADE)
      group = models.ForeignKey(Group, on_delete=models.CASCADE)
      date_joined = models.DateField()
  

• create an empty migration using python manage.py makemigrations users --empty command and create conversion script in python (more about the python migrations here) which creates new relations (Membership) for an old field (Group.members). It could look like this:

  # Generated by Django A.B on YYYY-MM-DD HH:MM
  import datetime
  
  from django.db import migrations
  
  
  def create_through_relations(apps, schema_editor):
      Group = apps.get_model('users', 'Group')
      Membership = apps.get_model('users', 'Membership')
      for group in Group.objects.all():
          for member in group.members.all():
              Membership(
                  person=member,
                  group=group,
                  date_joined=datetime.date.today()
              ).save()
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('myapp', '0005_create_models'),
      ]
  
      operations = [
          migrations.RunPython(create_through_relations, reverse_code=migrations.RunPython.noop),
      ]
  

• remove members field in the Group model and run python manage.py makemigrations, so our Group will look like this:

  class Group(models.Model):
      name = models.CharField(max_length=128)
  

• add members field the the Group model, but now with through property and run python manage.py makemigrations:

  class Group(models.Model):
      name = models.CharField(max_length=128)
      members = models.ManyToManyField(Person, through='Membership')
  

and that's it!

Now you need to change creation of members in a new way in your code - by through model. More about here.

You can also optionally tidy it up, by squashing these migrations.