Django 迁移错误: 不能在 M2M 字段中进行更改或从 M2M 字段中进行更改，也不能在 M2M 字段中添加或删除 through =

我试图修改一个 M2M 字段到一个 ForeignKey 字段。Valid命令显示没有任何问题，当我运行 syncdb 时:

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)

所以我不能迁徙了。

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)

有什么建议吗？

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小开

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?

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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.

小开

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'),
),
小开

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.

小开

This link helps you resolve all problems related to this The one which worked for me is python3 backend/manage.py migrate --fake "app_name"

小开

I literally had the same error for days and i had tried everything i saw here but still didn'y work. This is what worked for me:

  • I deleted all the files in migrations folder exceps init.py
  • I also deleted my database in my case it was the preinstalled db.sqlite3 After this, i wrote my models from the scratch, although i didn't change anything but i did write it again.
  • Apply migrations then on the models and this time it worked and no errors.
小开

I had the same problem and found this How to Migrate a ‘through’ to a many to many relation in Django article which is really really helped me to solve this problem. Please have a look. I'll summarize his answer here,

There is three model and one(CollectionProduct) is going to connect as many-to-many relationship.

enter image description here

This is the final output,

class Product(models.Model):
pass




class Collection(models.Model):
products = models.ManyToManyField(
Product,
blank=True,
related_name="collections",
through="CollectionProduct",
through_fields=["collection", "product"],
)




class CollectionProduct(models.Model):
collection = models.ForeignKey(Collection, on_delete=models.CASCADE)
product = models.ForeignKey(Product, on_delete=models.CASCADE)


class Meta:
db_table = "product_collection_products"

and here is the solution,

The solution

Take your app label (the package name, e.g. ‘product’) and your M2M field name, and combine them together with and underscore:

APPLABEL + _ + M2M TABLE NAME + _ + M2M FIELD NAME

For example in our case, it’s this:

product_collection_products

This is your M2M’s through database table name. Now you need to edit your M2M’s through model to this:

Also found another solution in In Django you cannot add or remove through= on M2M fields article which is going to edit migration files. I didn't try this, but have a look if you don't have any other solution.

小开

Another approach that worked for me:

  • Delete the existing M2M field and run migrations.
  • Add the FK field and run migrations again.

FK field added in this case has no relation to the previously used M2M field and hence should not create any problems.

小开

This worked for Me as well

  1. Delete last migrations
  2. run command python manage.py migrate --fake <application name>
  3. run command 'python manage.py makemigrations '
  4. run command 'python manage.py migrate'

Hope this will solve your problem with deleting database/migrations

小开

  1. First delete the migrations in your app (the folders/ files under 'migrations' folder) Showing the 'migrations' folder

  2. Then delete the 'db.sqlite3' file Showing the 'db.sqlite3' file

  3. And run python manage.py makemigrations name_of_app

  4. Finally run python manage.py migrate

小开

this happens when adding 'through' attribute to an existing M2M field: as M2M fields are by default handled by model they are defined in (if through is set). although when through is set to new model the M2M field is handled by that new model, hence the error in alter solutions:- you can reset db or remove those m2m fields and run migration as explained above then create them again

小开

*IF YOU ARE IN THE INITIAL STAGES OF DEVELOPMENT AND CAN AFFORD TO LOOSE DATA :)

delete all the migration files except init.py

then apply the migrations.

python manage.py makemigrations
python manage.py migrate

this will create new tables.


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