Eager Loading Strategies

Introduction

Rails provides three methods for eager loading associations: includes, preload, and eager_load. Each uses a different SQL strategy, and choosing the right one depends on whether you need to filter or just display associated data.

Key Concepts

• includes: Smart method that lets Rails choose between separate queries or a JOIN based on your query conditions.
• preload: Always uses separate queries — one per association.
• eager_load: Always uses a single LEFT OUTER JOIN query.

Real World Context

In a production dashboard showing orders with customers and line items, choosing preload over eager_load can cut memory usage in half by avoiding massive JOIN result sets. The right strategy depends on your query pattern.

Deep Dive

includes (Default Choice)

Rails automatically picks the best strategy:

ruby
# Two separate queries (default behavior)
Post.includes(:author)
# SELECT * FROM posts
# SELECT * FROM authors WHERE id IN (...)

# Single JOIN when filtering on the association
Post.includes(:author).where(authors: { active: true })
# SELECT * FROM posts LEFT OUTER JOIN authors ON ...
# WHERE authors.active = true

preload (Always Separate Queries)

ruby
Post.preload(:author, :comments)
# Query 1: SELECT * FROM posts
# Query 2: SELECT * FROM authors WHERE id IN (...)
# Query 3: SELECT * FROM comments WHERE post_id IN (...)

eager_load (Always JOIN)

ruby
Post.eager_load(:author)
# SELECT posts.*, authors.*
# FROM posts
# LEFT OUTER JOIN authors ON authors.id = posts.author_id

Nested Eager Loading

ruby
Post.includes(comments: :author)
Post.includes(:author, :category, :tags)
Post.includes(:author, comments: [:author, :likes], tags: :category)

Common Pitfalls

1. Using eager_load for display-only pages — JOINs create large result sets with duplicated data. Use preload when you just need to display associated records.
2. Forgetting nested associations — Loading Post.includes(:comments) but then accessing comment.author in the view still causes N+1. Include the full chain: includes(comments: :author).

Best Practices

1. Start with includes — It handles most cases correctly. Only switch to preload or eager_load when you have a specific reason.
2. Use strict_loading in development — Enable strict_loading to raise errors on lazy-loaded associations: Post.strict_loading.includes(:author).

Summary

• includes is the default — Rails picks the best SQL strategy automatically.
• preload forces separate queries, ideal for large data sets.
• eager_load forces a JOIN, required when filtering by association columns.
• Always include nested associations you access in views.

# Let Rails choose the strategy
Post.includes(:author).where(authors: { verified: true })

# Force separate queries for large datasets
Post.preload(:author, :comments).limit(100)

# Force JOIN when you must filter by association
Post.eager_load(:author).order('authors.name ASC')