When is better to use preload or eager_load vs includes?

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.

If you don’t know the difference you can read… What is the difference between includes, preload, eager_load and joins?

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