How does Django implement the mechanism of preloading related objects using prefetch_related?

In Django, the prefetch_related method is used to optimize database queries when you need to retrieve related objects (for example, related through ForeignKey, ManyToMany, etc.).

The mechanism works as follows: Django performs a separate SQL query for related objects and then links them with the main objects in Python code. This helps avoid the N+1 query problem, where a separate query is made to the related table for each main object.

Example:

# Suppose there are models Author and Book, where an author has many books
authors = Author.objects.prefetch_related('book_set').all()
for author in authors:
    books = list(author.book_set.all())  # books are already loaded, no additional queries

Thus, prefetch_related executes two queries: one for authors, and another for all books related to these authors, then links the data in memory.