Common ActiveRecord Pitfalls that Impede Rails App Performance

ActiveRecord is a powerful ORM (Object-Relational Mapping) tool widely used in Rails applications. It allows developers to interact with the database without writing raw SQL queries, making creating, reading, updating, and deleting records easy. However, like any tool, ActiveRecord is not infallible, and if not used correctly, it can slow down your Rails application. This blog will discuss some common ActiveRecord mistakes that can slow down your Rails app. 

1. N+1 queries

One of the most common ActiveRecord mistakes that can slow down your Rails app is N+1 queries. N+1 queries occur when you retrieve a collection of objects and then iterate over that collection, performing a database query for each object. For example, if you have a User model and a Blog model, and you want to retrieve all the blogs for each user, you might write code like this: 

users = User.all 

users.each do |user| 

    blogs = user.blogs 

end 

This code will generate N+1 queries, where N is the number of users. The first query retrieves all the users, and then each subsequent query retrieves the blogs for a single user. This can be very slow if you have many users. To avoid N+1 queries, you can use the `includes` method to eager-load the associated records: 

users = User.includes(:blogs) 

users.each do |user| 

    blogs = user.blogs 

end 

This code will generate only two queries: one to retrieve all the users and one to retrieve all the blogs for those users. This is much more efficient than N+1 queries. 

 2. Using `where` with associations

Another common ActiveRecord mistake that can slow down your Rails app is using the `where` method with associations. For example, if you have a User model and a Blog model, and you want to retrieve all the blogs written by a particular user, you might write code like this: 

user = User.find(1) 

blogs = user.blogs.where(published: true) 

This code will generate a SQL query that joins the users’ table with the blogs table and applies a WHERE clause to the blogs table. While this works, it can be slow if you have many blogs. To avoid this, you can use a subquery: 

user = User.find(1) 

blogs = Blog.where(user_id: user.id, published: true) 

This code will generate a SQL query that selects all the blogs where the user_id equals 1 and the published attribute is true. This is much more efficient than using `where` with associations. 

3. Overusing callbacks

ActiveRecord callbacks are hooks that allow you to trigger code at specific points in the lifecycle of a record. For example, you can use the `before_save` callback to run some code before a record is saved to the database. While callbacks can be useful, they can also slow down your Rails app if you overuse them. This is because callbacks are executed every time a record is saved, even if you don’t need them. To avoid overusing callbacks, you should only use them when necessary and ensure they are as efficient as possible. 

 4. Using `update_all` with conditions

The `update_all` method is a fast way to update multiple records in the database at once. However, if you use `update_all` with conditions, it can slow down your Rails app. If the conditions are too broad, update_all can update many records, potentially causing a performance bottleneck. Additionally, if there are any database triggers or callbacks associated with the affected records, they may not be triggered when using update_all 

 5. Unused Scopes

Scopes are a powerful ActiveRecord feature that allows you to define and reuse common queries throughout your application. However, they can slow down your application if you define too many scopes and don’t use them. Make sure only to represent the needed scopes and remove any unused ones. 

6. Large Associations

Associations in ActiveRecord can be convenient but can also slow down your application if they become too large. For example, if you have a User model that has_many: blogs, and each blog has_many: comments, then fetching all the comments for a user can be slow if they have many blogs. In this case, you may consider fetching comments from a separate query instead of the association. 

7. Unoptimized Database Queries

Sometimes, Active Record generates suboptimal SQL queries that can slow down your application. For example, if you use the where method with a string instead of a hash, ActiveRecord will generate a subquery for each item in the collection. To avoid this, always use a hash with the where method. 

# Suboptimal 

@blogs = Blog.where("published_at >=?", 1.week.ago) 

# Optimized 

@blogs = Blog.where("published_at >=?", 1.week.ago.to_date) 

 8. Not Using Database Indexes

Database indexes can significantly speed up queries, but they are often overlooked. Add indexes to your database tables for the columns you frequently search or order by. 

Active Record is a powerful tool, but it’s essential to use it correctly to avoid slowing down your Rails application. By following these tips, you can optimize your database queries and improve the performance of your application. 

Author: Sonali Barkund