Enum vs. Active Record any?

Today at work I saw a really weird exception in prod. The code is in a Rails project using ActiveRecord and was something like this:

if some_things_fetched_from_db.any?
  some_things_fetched_from_db.first.a_method
else
  another_value
end

And the dreaded exception that makes every rubyist want to cry:

NoMethodError: undefined method `a_method' for nil:NilClass

If there are any? of my things how can the first one be nil?

When I saw this I had to remind myself of the best debugging advice I ever got: There has to be a reason. Computers are just machines, they just follow instructions. If the code after if was executed, the condition before it had to be true. But [nil].any? is false, so what’s up?

It turns out calling any? on an ActiveRecord::Relation is not the same thing as calling any? on a regular ruby Enumerable thing. Who knew?

If you call any? on an active record relation without a block it just runs a COUNT on the collection of things it would return if it were to actually return any things, since ActiveRecord::Relations are lazy. They only ever really get stuff from the database when the data they represent is absolutely necessary. Until then they are just an object that knows how to fetch the things they were originally asked to fetch.

This causes problems if the things that are supposed to be in the relation could possibly be gone by the time you need them. Unlikely, yes, but possible, obviously.

ActiveRecord’s lazy loading caused a race condition. So [the_thing_i_thought_was_here].any? returned true, but by the time [the_thing_i_thought_was_here].first was executed, the_thing_i_thought_was_here had been deleted by another job spawned by a callback running in another process accessing the same database. Because sometimes good people write bad code.

So, next time something looks impossible and makes no sense, remember:

1. Things are not always what they seem.
2. ActiveRecord::Relation s are lazy.

Happy coding!