Django migration error :you cannot alter to or from M2M fields, or add or remove through= on M2M fields

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 i         


        

Answer 1

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.

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.