Lazy loading vs. eager loading in Ruby on Rails
In any web application, database performance plays a crucial role in determining the overall user experience. Efficiently retrieving data from the database can make a significant difference in response times and, consequently, the responsiveness of our application. In Rails, two key strategies for handling database queries are lazy loading and eager loading.
Lazy loading
Lazy loading, also known as on-demand loading, is a technique where data is loaded from the database only when it is specifically requested. In the context of
Example
Consider a simple scenario where we have two models: User and Groups, where each user can be a part of many groups and each group can have many users. There exists a third Expense model that is associated with both the User and Group models and has an association table, expenses.
# User Modelclass User < ApplicationRecordhas_many :expenseshas_many :groups, through: :expensesend# Group Modelclass Group < ApplicationRecordhas_many :expenseshas_many :users, through: :expensesend# Expense Modelclass Expense < ApplicationRecordbelongs_to :userbelongs_to :groupend
Now, if we fetch a user, the associated groups will not be loaded by default:
user = User.find(1)user.groups # This triggers a new database query to retrieve the groups.
Note: Lazy loading is the default behavior in Rails for associated records.
Eager loading
Eager Loading, on the other hand, is a strategy where associated data is loaded from the database upfront, along with the initial query. This reduces the number of database queries required to retrieve the necessary data.
Example
In the same scenario, if we want to fetch a user and their associated groups in a single query, we can use eager loading:
user = User.includes(:groups).find(1)user.groups # Now, this will not trigger a new database query to retrieve the groups.
In this example, User.includes(:groups) instructs ActiveRecord to load the associated groups for the user along with the initial query. This means that no additional database queries are needed when accessing user.groups.
Try it yourself
Load the terminal given below and try the above-mentioned example yourself. Notice that two separate queries will be run in the case of lazy loading, while in the case of eager loading, all the queries will be run beforehand, and we can fetch the groups associated with a user without running a database query.
When to use each strategy
Lazy loading | Eager loading |
Dealing with large datasets to avoid loading associated records upfront could lead to unnecessary data retrieval. | Needing the associated records and wanting to minimize the number of database queries. |
Wanting to minimize memory usage and only load data when needed. | Rendering views that rely on associations and wanting to avoid N+1 query issues. |
N+1 query problem
One of the primary considerations when choosing between lazy loading and eager loading is the N+1 query problem. This occurs when we fetch a collection of records (e.g., all users) and then need to access an associated record for each of them (e.g., their posts).
With lazy loading, this would result in a separate database query for each associated record, leading to a potentially large number of queries. Eager loading addresses this by loading all associated records upfront, eliminating the need for additional queries.
Conclusion
Choosing between lazy loading and eager loading in Rails depends on the specific requirements of the application. While lazy loading conserves resources and can be more memory-efficient, eager loading is often necessary to avoid N+1 query issues and improve performance when we know we’ll need associated data.
Understanding the strengths and weaknesses of both strategies will empower us to make informed decisions about database query optimization in our Rails application, ultimately enhancing the user experience.
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