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# What is the difference between join and include?

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In Rails, joins and includes are both used to handle associations between models, but they serve different purposes and have different behaviours.

Joins

  • Purpose: Used to perform SQL joins between tables.
  • Behavior: Executes a single SQL query with an INNER JOIN, which combines rows from two or more tables based on a related column.
  • Use Case: Ideal for filtering records based on associated table conditions.
  • Example:
# Find all users who have posts
User.joins(:posts).where(posts: { published: true })
This generates a SQL query that joins the users and posts tables and filters users based on the condition that their posts are published.

Includes

  • Purpose: Used for eager loading associations to avoid the N+1 query problem.
  • Behavior: Executes multiple SQL queries to load the associated records and then combines them in memory.
  • Use Case: Ideal for loading associated records to avoid multiple database queries.
Example:

# Load users and their posts
users = User.includes(:posts)
users.each do |user|
  user.posts.each do |post|
    puts post.title
  end
end
This generates two SQL queries: one to load users and another to load their associated posts, avoiding the N+1 query problem

Key Differences
  • Query Execution: joins performs a single query with an INNER JOIN, while includes performs multiple queries to load associated records.
  • Use Case: joins is used for filtering based on associated records, while includes is used for eager loading to avoid N+1 queries.
  • Performance: joins can be more efficient for filtering, but includes is better for loading associated data without multiple queries.
Combined Use

You can also combine joins and includes to leverage both filtering and eager loading:

# Find users with published posts and eager load their posts
User.joins(:posts).includes(:posts).where(posts: { published: true })
Understanding when to use joins versus includes can help optimize your application’s performance and ensure efficient database interactions.