'Regression test for #12209 -- If the explicitly provided through model
is specified as a string, the admin should still be able use
Model.m2m_field.through'

def test_explicit_through_override(self):

    class AuthorsInline(admin.TabularInline, ):
        model = Book.authors.through
    class BookAdmin(admin.ModelAdmin, ):
        inlines = [AuthorsInline]
    BookAdmin.validate(Book)
