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Tag: predicate methods

Does your predicate return what you think it does?

def has_query?
  self.param_search_query && self.param_search_query != "Eventname"
end

I found this in some code I was working on today. This is a pretty standard “predicate method”, which, by convention, should always return either true or false. Strangely, as I was exploring the code with Pry, I noticed the method return nil.

It turned out that param_search_query was actually nil, which, when mixed with the logical AND (&&) operator returns something the author clearly did not expect:

mike@sleepycat:~☺  irb
irb(main):001:> nil && true
=> nil

This was actually surprising to me too. I would have thought that nil would have been falsey and that the expression would return false. Definitely something I am going to remember next time I am using the && operator.

So the next question is, what to do about it.
One way to ensure a boolean return value that I have seen is to use the bang operator twice. This works because !nil is true and !!nil is false:

irb(main):003:0> !!nil
=> false

So we could use that to fix the method like so:

def has_query?
  !!(self.param_search_query && self.param_search_query != "Eventname")
end

I’m not a fan of this because I think it obscures the intention of the author.
In the end, since this was in a Rails app I went with the present? method:

def has_query?
  self.param_search_query.present? && self.param_search_query != "Eventname"
end

The tests pass and I’m back to hunting bigger game…