How are symbols used to identify arguments in ruby methods

Question

I am learning rails and going back to ruby to understand how methods in rails (and ruby really work). When I see method calls like:

validates :first_name, :presen         


        

Answer 1

Symbols and hashes are values like any other, and can be passed like any other value type.

Recall that ActiveRecord models accept a hash as an argument; it ends up being similar to this (it's not this simple, but it's the same idea in the end):

class User
  attr_accessor :fname, :lname

  def initialize(args)
    @fname = args[:fname] if args[:fname]
    @lname = args[:lname] if args[:lname]
  end
end

u = User.new(:fname => 'Joe', :lname => 'Hacker')

This takes advantage of not having to put the hash in curly-brackets {} unless you need to disambiguate parameters (and there's a block parsing issue as well when you skip the parens).

Similarly:

class TestItOut
  attr_accessor :field_name, :validations

  def initialize(field_name, validations)
    @field_name = field_name
    @validations = validations
  end

  def show_validations
    puts "Validating field '#{field_name}' with:"
    validations.each do |type, args|
      puts "  validator '#{type}' with args '#{args}'"
    end
  end
end

t = TestItOut.new(:name, presence: true, length: { min: 2, max: 10 })
t.show_validations

This outputs:

Validating field 'name' with:
  validator 'presence' with args 'true'
  validator 'length' with args '{min: 2, max: 10}'

From there you can start to see how things like this work.