N+1 Queries Problem in Rails

In Ruby on Rails applications, the N+1 query problem often sneaks in during the development of ActiveRecord associations. This issue can drastically affect the performance of your application by making an excessive number of database queries, which can slow down your application. But what exactly are N+1 queries, and how can you solve them? Let's break it down.

What are N+1 Queries?

The N+1 query problem occurs when your application makes 1 query to retrieve the primary objects, and then N additional queries to fetch associated objects for each primary object. Here, "N" represents the number of primary objects retrieved by the initial query, and "1" signifies the initial query itself.

Example of N+1 Queries

Consider a blogging platform where each Post has many Comments. If you want to display all posts along with their comments, you might do something like this:

@posts = Post.limit(10)
@posts do |post|
  puts post.comments.text
end

And in your view:

<% @posts.each do |post| %>
  <h2><%= post.title %></h2>
  <% post.comments.each do |comment| %>
    <p><%= comment.body %></p>
  <% end %>
<% end %>

This innocent-looking code leads to the N+1 query problem. Here's why:

• 1 query is made to fetch all posts.
• N queries are made to fetch comments for each post (if there are 10 posts, 10 additional queries are made to fetch comments for each post).

How to Fix N+1 Queries

Active Record lets you specify in advance all the associations that are going to be loaded.

The methods are:

• includes: With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.
• preload: With preload, Active Record loads each specified association using one query per association.
• eager_load: With eager_load, Active Record loads all specified associations using a LEFT OUTER JOIN.

Use Join

Ruby on Rails ActiveRecord provides a .joins method that can be used to address N+1 query problems in certain situations, particularly when you want to filter or sort by fields on associated tables. Unlike includes, which is designed for preloading related records to avoid N+1 queries, .joins performs an SQL JOIN operation. This can be more efficient in cases where you don't need to access all the attributes of the related records, but rather need to query based on them or include them in the select statement.

@posts_with_comments = Post.joins(:comments).distinct

This query fetches posts that have at least one comment by performing an INNER JOIN between posts and comments. The distinct method is used to ensure each post is listed only once, even if it has multiple comments.

When to Use .joins

Use .joins when you need to filter or sort queries based on associated records' attributes.
It's also useful for aggregations or when counting related records.

Limitations

While .joins can help avoid N+1 queries by allowing you to filter or aggregate data based on associated records, it doesn't automatically preload these associated records for later use. Accessing unloaded associations will trigger separate queries, potentially leading to N+1 issues if not managed carefully.

Solution

For scenarios where you need to both join and preload associated records to avoid N+1 queries while also using the data for filtering or sorting, you can combine .joins with .includes:

@posts = Post.joins(:comments).includes(:comments).distinct

This approach leverages .joins to filter or sort based on the associated table and .includes to preload the associated records, ensuring efficient data access and avoiding N+1 queries.

Understanding when and how to use .joins, .includes, and other ActiveRecord query methods like .preload and .eager_load is essential for optimizing database queries in Ruby on Rails applications. Each method serves different purposes, and choosing the right one depends on your specific requirements for querying and accessing associated records.