ManyToManyFields using through won't respect to_field parameter in ForeignKey fields

[aitorciki]

When a ManyToManyField is set to use an existing model with through, Django will fail to generate the correct SQL if a ForeignKey in this model is using to_field to relate to a field other than the primary key of the target table. That happens because m2m handling code is always assuming that the relation if with the target table primary key.

For models like the following:

class Author(models.Model):
    name = models.CharField(max_length=50, unique=True)

class Book(models.Model):
    title = models.CharField(max_length=50, unique=True)
    authors = models.ManyToManyField(Author, through='BookToAuthor')

class BookToAuthor(models.Model):
    author = models.ForeignKey(Author, db_column='author_name', to_field='name')
    book = models.ForeignKey(Book, db_column='book_title', to_field='title')

The generated SQL for an book object (id = 1) access to authors attribute is

SELECT `author`.`id`, `author`.`name` FROM `author` INNER JOIN `booktoauthor` ON (`author`.`id` = `booktoauthor`.`author_name`) WHERE `booktoauthor`.`book_title` = 1

while the expected one is

SELECT `author`.`id`, `author`.`name` FROM `author` INNER JOIN `booktoauthor` ON (`author`.`name` = `booktoauthor`.`author_name`) INNER JOIN `book` ON (`booktoauthor`.`book_title` = `book`.`title`) WHERE `book`.`id` = 1

I attach a patch that uses the relation field_name attribute instead of assuming that the primary key is to be used.

[Ramiro Morales]

Duplicate of #13343 that was closed in favor of #11319. I will add a note there pointing to this ticket.