Even if you already know what is the difference between includes, preload and eager_load, maybe is not that clear when is better for you to use one vs the other.

Well, as you may know, with Rails/ActiveRecord you can preload associations in 2 different ways:

• With a LEFT OUTER JOIN to pull all the associations in one query (eager_load).
• Making a separate query for each association (preload)

And when you use includes Rails will decide which strategy to use for you. I think that this is a good default.

But sometimes you already know (by using a performance measuring tool) that one strategy is significantly slower than the other…

That’s a good time to use preload or eager_load directly, to force explicitly use the strategy that you prefer.

Do you want to practice?

Practice will help you understand this concepts better. I have prepared some examples/excercises to help you understand how you can use this methods. Give it try!

Examples to learn the difference between preload, includes or eager_load

Related articles

• 5 ways to fetch the latest-N-of-each record on Rails
• Association with default order, to fix the latest-comment n+1 problem
• Cheatsheet to help you decide which fix for the last-comment n+1 problem is the best for you
• Examples to learn the difference between preload, includes or eager_load