Why Rails can use `if` as hash key but not in Ruby

Question

In pure Ruby irb, one cannot type {if: 1}. The statement will not terminate, because irb thinks if is not a symbol but instead the beginning of an

Answer 1

The code in your example is part of the Rails DSL. What you are actually setting there is a hash which just happens to look a bit like code.

Internally, Rails will evaluate this hash specifying conditions to the before_save call.

In a very simplified version, Rails basically does this when saving:

class ActiveRecord::Base
  @before_save_rules = []
  def self.before_save(method, options={})
    @before_save_rules << [method, options]
  end

  def self.before_save_rules
    @before_save_rules
  end

  def save
    # Evaluate the defined rules and decide if we should perform the
    # before_save action or not

    self.class.before_safe_rules.each do |method, options|
      do_perform = true
      if options.key?(:if)
        do_perform = false unless send(options[:if])
      end

      if options.key?(:unless)
        do_perform = false if send(options[:unless])
      end

      send(method) if do_perform
    end

    # now perform the actual save to the database
    # ...
  end
end

Again, this is very simplified and just in the spirit of actual code, but this is basically how it works.