(Newbie) Stuck with annotate() docs

Folks, I'm having exceptional trouble understanding annotate(), aggregate(), and their various combinations. I'm currently stuck here: https://docs.djangoproject.com/en/1.9/topics/db/aggregation/#combining-multiple-aggregations

The example here uses Book.objects.first().chapters.count(), but there's no chapters model or field at the start of the tutorial. It's frustrating, to say the least. Even if I set up a separate application to test this myself, what do I make of "chapters"? Is it another model with many-to-many relation with Book? When I ran an example with the following models:

class Author(models.Model):

    name = models.CharField(max_length=100)

    age = models.IntegerField()

class Book(models.Model):

    name = models.CharField(max_length=300)

    chapters = models.IntegerField()

    authors = models.ManyToManyField(Author)

I got: 

>>> Book.objects.first().chapters.count()

Traceback (most recent call last):

  File "<console>", line 1, in <module>

AttributeError: 'int' object has no attribute 'count'

>>> 

So basically, I feel like I'm screwed. Please help.

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