Why do callbacks use symbols in Ruby on Rails

Question

I am struggling to understand when and when not to use symbols in Rails. I understand that symbols are not too dissimilar from a string without many of the methods. I also understand the symbols make good keys as symbols of the same name occupy one address in memory.

What I struggle to understand is why Rails decides to use symbols in some cases. If I had the callback

before_action :ask_stack_overflow_question

def ask_stack_overflow_question
  puts "why did I just use a symbol?"
end

I don't quite understand why the treat the method as a symbol? Why would I need to keep a method constant?

I can't find any answer to this in the documentation.

Thank you

Answer 1

This isn't as tricky as it seems.

When calling before_action you do not want the method to be executed immediately, but you have to describe that action you want to have performed somehow. A symbol is used here to represent the name of the method to call.

So if this is captured as name, then later, somewhere in the code, this is converted into a send(name) at the appropriate time.

There's a big difference between these two. The immediate version is:

before_action method_name

Which has the effect of executing method_name immediately, as Ruby will be compelled to run method_name to find out what it returns, then pass that through to before_action as an argument.

The deferred convention is:

before_action :method_name

Which is simply a polite request to before_action, passing in a symbol as an argument. You're leaving it up to before_action to deal with this however it sees fit, so it may or may not execute that method in the future, it depends on the situation.

This is how it works within Ruby with very few exceptions. The only one I can think of that doesn't immediately execute a method when it's specified immediately instead of as a representative symbol is the quirky alias construct.