Fix: ValueError: Cannot alter field <model>.<field> into <model>.<field> - they are not compatible types (you cannot alter to or from M2M fields, or add or remove through= on M2M fields)

django_migrate_m2m_add_through_model.py

# Changing the through= part of a models.ManyToManyField() fails to migrate, 
# as Django won't let us do this in a single step. We can manually workaround 
# with several migration steps though, as shown below.
#
# Based on Django 3.2

# 1) Initial state of models.py

from django.db import models

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

class Store(models.Model):
    city = models.CharField(max_length=128)
    products = models.ManyToManyField(Product, related_name="stores", blank=True)

# 2) Add a through model, e.g. as you want to track Products being sold out in a Store

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

class Store(models.Model):
    city = models.CharField(max_length=128)
    products = models.ManyToManyField(
        Product, related_name="stores", blank=True, through="StoreProduct"
    )

class StoreProduct(models.Model):
    store = models.ForeignKey("Store", on_delete=models.CASCADE)
    product = models.ForeignKey("Product", on_delete=models.CASCADE)
    sold_out = models.BooleanField(default=False)

# 3) Try running makemigrations then migrate, it should fail like this:

"""
ValueError: Cannot alter field core.Store.products into core.Store.products - 
  they are not compatible types (you cannot alter to or from M2M fields, or 
  add or remove through= on M2M fields)
"""

# 4) Revert your latest changes in models.py and remove the last migration file. 
# Instead we add a copy of the original m2m field, copy the original mappings, 
# then remove the old field, then rename the new field.

class Store(models.Model):
    city = models.CharField(max_length=128)
    products = models.ManyToManyField(Product, related_name="stores", blank=True)
    products2 = models.ManyToManyField(
        Product, related_name="stores2", blank=True, through="StoreProduct"
    )

class StoreProduct(models.Model):
    store = models.ForeignKey("Store", on_delete=models.CASCADE)
    product = models.ForeignKey("Product", on_delete=models.CASCADE)
    sold_out = models.BooleanField(default=False)

# 5) Run makemigrations and extend the migration manually like so:

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

def copy_store_products_to_new_field(apps, schema_editor):
    Store = apps.get_model("core", "Store")
    for store in Store.objects.filter(products__isnull=False):
        store.products2.set(store.products.all())

class Migration(migrations.Migration):

    dependencies = [
        ("core", "0001_initial"),
    ]

    operations = [
        migrations.CreateModel(
            name="StoreProduct",
            fields=[
                (
                    "id",
                    models.BigAutoField(
                        auto_created=True,
                        primary_key=True,
                        serialize=False,
                        verbose_name="ID",
                    ),
                ),
                ("sold_out", models.BooleanField(default=False)),
                (
                    "product",
                    models.ForeignKey(
                        on_delete=django.db.models.deletion.CASCADE, to="core.product"
                    ),
                ),
                (
                    "store",
                    models.ForeignKey(
                        on_delete=django.db.models.deletion.CASCADE, to="core.store"
                    ),
                ),
            ],
        ),
        # Copy the original field, but assign our new model as through=
        migrations.AddField(
            model_name="store",
            name="products2",
            field=models.ManyToManyField(
                blank=True,
                related_name="stores2",
                through="core.StoreProduct",
                to="core.product",
            ),
        ),
        # Copy product associations from old field to new field
        migrations.RunPython(
            copy_store_products_to_new_field, migrations.RunPython.noop
        ),
        # Remove old field
        migrations.RemoveField(model_name="store", name="products"),
        # Rename new field
        migrations.RenameField(
            model_name="store", old_name="products2", new_name="products"
        ),
        # Use original related name
        migrations.AlterField(
            model_name="store",
            name="products",
            field=models.ManyToManyField(
                blank=True,
                related_name="stores",
                through="core.StoreProduct",
                to="core.Product",
            ),
        ),
    ]

# 6) Before running that migration, make sure to update Store in models.py to match it:

class Store(models.Model):
    city = models.CharField(max_length=128)
    products = models.ManyToManyField(
        Product, related_name="stores", blank=True, through="StoreProduct"
    )

# 7) Run migrate, you should end up with an m2m field named 'products' using existing
# associations, but also now your new through= model.