`

# What is the difference between include, preload and eager_load?

178

In Rails, includes, preload, and eager_load are methods used to handle associations and avoid the N+1 query problem. Here’s a breakdown of each:

includes

  • Purpose: Loads the specified associations to avoid N+1 queries.
  • How it works: By default, it behaves like preload, but if conditions are added to the query, it can act like eager_load.
Example:
posts = Post.includes(:comments)
posts.each do |post|
  puts post.comments.size
end

preload

  • Purpose: Loads the specified associations in separate queries.
  • How it works: Executes one query to load the primary model and another to load the associated models.
Example:
posts = Post.preload(:comments)
posts.each do |post|
  puts post.comments.size
end
This will also load all posts and their associated comments in two separate queries

eager_load

  • Purpose: Loads the specified associations using a single query with a LEFT OUTER JOIN.
  • How it works: Executes a single query to load both the primary and associated models.
Example:
posts = Post.eager_load(:comments)
posts.each do |post|
  puts post.comments.size
end
This will load all posts and their associated comments in one query using a LEFT OUTER JOIN

Summary
  • includes: Flexible, can act like preload or eager_load depending on the query.
  • preload: Always uses separate queries, better for memory usage.
  • eager_load: Uses a single query with a JOIN, can be more efficient but may use more memory.
Each method has its use case depending on the specific needs of your query and application performance considerations.