ManyToManyField.through does not work as the model property of an inline when specified using a string

[David Gouldin]

Using the following models:

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    name = models.CharField(max_length=100)
    authors = models.ManyToManyField(Author, through='AuthorsBooks')

class AuthorsBooks(models.Model):
    author = models.ForeignKey(Author)
    book = models.ForeignKey(Book)

... with this admin:

class AuthorsInline(admin.TabularInline):
    model = models.Book.authors.through

class BookAdmin(admin.ModelAdmin):
    inlines = [AuthorsInline]
    exclude = ('authors',)

admin.site.register(models.Book, BookAdmin)

... does not function. If you change the models to:

class AuthorsBooks(models.Model):
    author = models.ForeignKey('Author')
    book = models.ForeignKey('Book')

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    name = models.CharField(max_length=100)
    authors = models.ManyToManyField(Author, through=AuthorsBooks)

... the admin works just fine. This seems to be an issue with specifying Book.authors.through as a string instead of using the model class itself.

[Russell Keith-Magee]

(In [11736]) Fixed #12209 -- Made the through attribute on a m2m relation into a property to ensure that the fully resolved through model is always provdided. Thanks to dgouldin for the report.