Make AlterField a no-op when ManyToManyField's through.db_table remains the same between renames.

[Clifford Gama]

​As suggested by Simon. Similar to f05cc5e3d2ae7663dbd248029bcb74500cf1029f (for db_table on foreign keys) and #31825 for db_columns.

[Jacob Walls]

Thanks. #33197 is another similar fixed ticket.

[Clifford Gama]

This is likely not a feasible optimisation with the way the db_table name of an auto-generated through model ​is backend aware, while the auto detector and makemigrations are backend agnostic. In other words, we have nothing to compare the new db_table with at makemigrations time if the through model is auto-created.

Please reopen if I've missed something.

May be related to #33201.

[Clifford Gama]

On another note, I just found out that:

class Book(models.Model):
    pass


class Author(models.Model):
    books = models.ManyToManyField(Book, through="AuthorBook")


class AuthorBook(models.Model):
    book = models.ForeignKey(Book, models.CASCADE)
    author = models.ForeignKey(Author, models.CASCADE)

renaming books, to articles without changing anything else results in a RemoveField and AddField, though nothing has changed in the schema

[Clifford Gama]

We can still optimise when there is an explicit db_table. I opened a ​PR