The amount of times in my career where the performance would be even measurable between these 2 styles is vanishingly few. If I cared I could just use #filter_map or a classic for-each loop. If we're assuming the project is written in Ruby in the first place, clearly we're not valuing performance that highly.

The more important thing is the ruby version is chainable and more readable—and that matters far more often.

You had me until

> more readable

Without having more context, User in your code is possibly an ActiveRecord class, so to read the #select, you have to consider that it could be radically different and heavier than Enumerable#select (eg. it may execute a DB query as a side effect). So I like your example still, because it’s representative, and certainly expressive, but not readable per se. IMO this sin here is simply that ActiveRecord shouldn’t have overloaded select.

That’s more of an argument against ORMs which I realize my example used. I was mostly trying to convey using map/filter against POROs though.

Using an ORM was a poor choice on my part. I agree the magic there does harm readability since it makes it a lot harder to understand what it really is doing.

Ruby has had lazy iteration[1] for quite a while but as n here is only 10 there's really not much to loose in not using it.

In fact given that only a subset of users will be admin users for the second loop n is going to < 10 anyway.

I believe it was Rob Pike that actually said something along the lines of not bothering to optimise until you know that n is large[2].

1. https://ruby-doc.org/core-2.5.0/Enumerator/Lazy.html

2. https://users.ece.utexas.edu/~adnan/pike.html

Not sure about Ruby, but Java’s streams would do one loop only here, as they are lazy. Also, both solutions are O(n), so unless this is the hot loop of your program, or it iterates over some insane amount of entities, it is a completely meaningless microoptimization.