Data loss if a ManyToManyField is shadowed by Prefetch

[Ian Foote]

With two models:

class Book(Model):
    name = CharField(max_length=100)

class Author(Model):
    books = ManyToManyField(Book)

it is possible to create a prefetch query that loses data from Author.books:

poems = Book.objects.filter(name='Poems')
Author.objects.prefetch_related(
    Prefetch('books', queryset=poems, to_attr='books'),
)

When this queryset is evaluated, each Author's books is overridden by the poems queryset.