Annotations

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.

Example 2:

Assume each Book has a rating field.

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.

Counts how many books each author published per year.

Example 2:

Finds the most recently published book for each genre.

Annotation with Filters

Example 1:

Counts the number of fantasy books written by each author.