Fixed #12209 -- Made the through attribute on a m2m relation into a property...

    # ReverseManyRelatedObjectsDescriptor instance.

    def __init__(self, m2m_field):

        self.field = m2m_field

    def _through(self):

        # through is provided so that you have easy access to the through

        # model (Book.authors.through) for inlines, etc.

        self.through = m2m_field.rel.through

        # model (Book.authors.through) for inlines, etc. This is done as

        # a property to ensure that the fully resolved value is returned.

        return self.field.rel.through

    through = property(_through)

    def __get__(self, instance, instance_type=None):

        if instance is None:

    e = models.CharField(max_length=1)

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)

__test__ = {'API_TESTS':"""

>>> validate_inline(TwoAlbumFKAndAnEInline, None, Album)

# Regression test for #12203 -- If the explicitly provided through model

# is specified as a string, the admin should still be able use

# Model.m2m_field.through

>>> class AuthorsInline(admin.TabularInline):

...     model = Book.authors.through

>>> class BookAdmin(admin.ModelAdmin):

...     inlines = [AuthorsInline]

# If the through model is still a string (and hasn't been resolved to a model)

# the validation will fail.

>>> validate(BookAdmin, Book)

"""}