在Rails中澄清Ruby语法

Question

I'm learning Rails and in going through the official guides , I came across some code which I could not really understand the meaning of.

Case 1 -

class Person < ApplicationRecord
  validates :name, presence: true
end 

It looks to me that validates is a method that takes a symbol called :name as an argument. But then, what is presence ? Is it also a method? But if it is, what is the significance of the : right after presence . I understand that the value true is being set for presence , which serves as kind of a validation, requiring the presence of (in other words). But I'm not quite clear on the syntax.

It might also be possible that presence: true is just a hash, where :presence (the symbol) is the key, and true is the value.

Case 2 -

class Person < ApplicationRecord
  validates :terms_of_service, acceptance: true, message: 'must be abided'
end 

Again, validates is the method that takes a symbol :terms_of_service as an argument. But what about the rest? Is it a hash with 2 key-value pairs, somewhat like {acceptance: true, message: 'must be abided'} ?

And if it is indeed a hash, why is it tacked on to the validates method in each case? Why can't it be

validates :terms_of_service
acceptance: true, message: 'must be abided'

Thanks for the help!

Answer 1

That is the syntax for passing a hash to the method. What that is doing is the same thing as validates(:terms_of_service, {acceptance: true, message: 'must be abided'}) . It's a common way of passing extra options to a method.

Answer 2

In Ruby there's a strong tradition for passing in options as a Hash as the last argument, strong enough that this tradition became new feature borrowed from Python: Keyword arguments.

In classic Ruby the method would be defined as this:

def validates(*args)
  options = args.last.is_a?(Hash) ? args.pop : { }

  # args is a list of fields
end

In Ruby 2.3 you can do this:

def validates(*args, **options)
  # options is automatically any hash-style arguments if present
end

In Ruby 2.0+ you can also do this:

def validates(*args, acceptance: false, message: nil)
end

Where that defines options as first-class variables.

It's a common Ruby pattern, so it's good to understand what's going on here. Try writing your own methods that take options and you'll see how it plays out.