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)

Using includes by default

When you use includes Rails will decide which strategy to use for you. I think that this is a good default. Use includes and let rails decide for you.

But, to be honest, that is not what I do…

Using preload by default

I (at least for now) prefer to use preload explicitly by default, because that is also the default for includes, and I think that is easier to know what is going to happen just by watching the code.

I use eager_load just when I already know that is significantly faster than the preload.

So, what should you do?

I would say, “Don’t worry to much”, and just pick one of this two strategies.

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

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