EAGER LOADING

Eager loading in Rails is a technique used to optimize database queries by loading associated records of the objects returned by the query, thereby reducing the number of database calls. This is particularly useful when you have associations defined in your models and you want to avoid the "N+1 query problem."

What is the N+1 Query Problem?

“N+1 Queries” are a very common cause of repeated queries in Rails applications. This happens when you make a request for a single row in one table, and then make an additional request per element in a has_many relationship, usually in a loop.
Here’s an example:

Consider two models: Author and Book, where an Author has many Books.

class Author < ApplicationRecord 
  has_many :books 
 end 

class Book < ApplicationRecord 
  belongs_to :author 
end 

Scenario Without Eager Loading
When you fetch authors and then access their books, Rails performs an additional query for each author to get their books.

# Fetching all authors and their books 
authors = Author.all 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

This will result in one query to fetch the authors and multiple queries to fetch books for each author, leading to the N+1 query problem.

How Eager Loading Works

Eager loading addresses this problem by loading all the necessary data in as few queries as possible, usually through the use of includes, eager_load, or preload.

Using Eager Loading with includes

To avoid the N+1 query problem, you can use includes to preload the associated books when fetching authors.

# Fetching all authors and their books with eager loading 
authors = Author.includes(:books) 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

With includes, Rails performs a single query to fetch all authors and another single query to fetch all associated books.

Using Eager Loading with eager_load

eager_load forces Rails to use a SQL JOIN to load the associated records. This can be useful if you need to filter or sort based on the associated records.

# Fetching all authors and their books with eager loading using JOIN 
authors = Author.eager_load(:books) 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

This approach performs a single query with a LEFT OUTER JOIN.

Using Eager Loading with preload

preload is similar to includes, but it always uses separate queries for loading the associations.

# Fetching all authors and their books with eager loading using separate queries 
authors = Author.preload(:books) 

authors.each do |author| 
  puts author.books.pluck(:title) 
end 

preload is useful when you know that separate queries will be more efficient, for example, when fetching a large number of records.

Summary

Eager loading is a powerful tool in Rails that helps to optimize database access by pre-loading associations. Using includes, eager_load, or preload appropriately can significantly improve the performance of your application by reducing the number of database queries.

#rails #db #optimizing_queries