**Annotation** in Django ORM allows you to *attach additional information* to each object in a QuerySet. It's a powerful feature for augmenting the results of queries with calculated values, which can be aggregates computed over related objects or values derived from model fields.

### Basic Annotation
#### Example 1:
Assume each Book has a `pages` field.
This annotates each genre with the total number of pages of all books within that genre.
``` python
from django.db.models import Sum

genre_with_page_count = Genre.objects.annotate(total_pages=Sum('book__pages'))
```
#### Example 2:
Assume each Book has a `rating` field.
``` python
from django.db.models import Avg

author_with_avg_rating = Author.objects.annotate(avg_rating=Avg('book__rating'))
```
Adds an average rating for the books written by each author.

### Advanced Annotation Use Cases
#### Example 1: 
Assume each Book has a `publication_date` field.
``` python
from django.db.models import Count, F, Year

books_per_year = Author.objects.annotate(year=F('book__publication_date__year')).values('year').annotate(count=Count('book'))
```
Counts how many books each author published per year.

#### Example 2:
Finds the most recently published book for each genre.
``` python
from django.db.models import Max

latest_book_per_genre = Genre.objects.annotate(latest_book=Max('book__publication_date'))
```

### Annotation with Filters
#### Example 1:
Counts the number of fantasy books written by each author.
``` python
author_genre_count = Author.objects.filter(book__genre__name='Fantasy').annotate(fantasy_books_count=Count('book'))
```

What is the primary purpose of annotation in Django ORM?

What does the following annotation query accomplish? 

Aprende a construir aplicaciones utilizando el framework Django ORM. Este es el siguiente paso en la expansión de su conocimiento del desarrollo de Django.

A quick overview of the Django ORM. One of the advantages of Django is the Django ORM.

You will learn about Django's robust modeling system, including field data types for precise database structuring. The section also covers migrations for seamless schema updates and the powerful admin interface, providing an intuitive way to manage your application's data.

This essential section is your treasure map for navigating Create, Read, Update, and Delete (CRUD) operations. Effortlessly craft queries to add new records, fetch and explore data with finesse, seamlessly update existing records, and confidently delete what's no longer needed. Each example is a key, unlocking the power and elegance of Django ORM. 🌟🔍💻🚀

Explore the intricate world of database relationships in Django! This section delves into the core concepts of Many-to-One, One-to-One, and Many-to-Many relationships. Learn how to effectively model real-world scenarios in your database using Django's powerful ORM, with clear examples and practical use-cases to guide you.

After this section, you will be able to master Django's field lookups, Q and F objects for advanced queries, and Aggregation and Annotation for data summarization. You'll also learn techniques to minimize database hits, addressing the N+1 problem, and effectively using select and prefetch related.

This section delves into advanced Django concepts, including the Meta class for model customization, the implementation of constraints for data integrity, the basics of transactions for ensuring database consistency, and an overview of the User model, a core component for handling user authentication and permissions in Django applications.

Django ORM Ninja: Técnicas Avanzadas para Desarrolladores

Annotations

Basic Annotation

Example 1:

Example 2:

Advanced Annotation Use Cases

Example 1:

Example 2:

Annotation with Filters

Example 1:

1. What is the primary purpose of annotation in Django ORM?

2. What does the following annotation query accomplish?