[Question] Explanation of books plural from book?

[scheung38]

Questions

Can someone please explain where we get the plural for book, books? From the docs:

https://django-elasticsearch-dsl-drf.readthedocs.io/en/latest/quick_start.html#full-database-sync:

search_indexes/signals.py

@receiver(post_save)
def update_document(sender, **kwargs):
    """Update document on added/changed records.

    Update Book document index if related `books.Publisher` (`publisher`),
    `books.Author` (`authors`), `books.Tag` (`tags`) fields have been updated
    in the database.
    """
    app_label = sender._meta.app_label
    model_name = sender._meta.model_name
    instance = kwargs['instance']

    if app_label == 'book':
        # If it is `books.Publisher` that is being updated.
        if model_name == 'publisher':
            instances = instance.books.all()
            for _instance in instances:
                registry.update(_instance)

i.e for my use case if I have instead of book, address then

instances = instance.addresses.all()

But where is the plural addresses defined? If defined here: books.Author first parameter:

authors = models.ManyToManyField('books.Author', related_name='books')

then if based on first or second parameter ? Since using Djongo.models there is no related_name key