Ruby Meta Programming

README.md

Ruby Meta Programming

Here you will see divers code snippets about meta programming with Ruby.

Hook called methods that not exists

The first snippet here shows a example the method callback named method_exists(). This method is trigged when a method that not exists in one class is executed. The implementation here use define_method(name) to create a new method in runtime!!

Gemfile

source 'https://rubygems.org'

gem 'rspec', require: false, group: :test
gem 'simplecov', require: false, group: :test

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    diff-lcs (1.2.5)
    docile (1.1.5)
    json (1.8.2)
    rspec (3.2.0)
      rspec-core (~> 3.2.0)
      rspec-expectations (~> 3.2.0)
      rspec-mocks (~> 3.2.0)
    rspec-core (3.2.3)
      rspec-support (~> 3.2.0)
    rspec-expectations (3.2.1)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.2.0)
    rspec-mocks (3.2.1)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.2.0)
    rspec-support (3.2.2)
    simplecov (0.10.0)
      docile (~> 1.1.0)
      json (~> 1.8)
      simplecov-html (~> 0.10.0)
    simplecov-html (0.10.0)

PLATFORMS
  ruby

DEPENDENCIES
  rspec
  simplecov

method_missing.rb

# This is a meta programming example of usage
# the method_missing feature that intercept
# unexistent methods of a class
class MyClass

  def method_missing(name, *args)
    method_name = name.to_s

    if is_method? method_name

      self.class.send(:define_method, method_name) do |value|

        field = method_name.gsub(/find_by_/,'')
        return "This method find by '#{field}' with value: '#{value}'"

      end

      self.send(name,args[0])

    elsif
      super(name, args)
    end

  end

  def respond_to?(symbol, include_private=false)
    is_method?(symbol.to_s) ? true : super(symbol, include_private)
  end

  protected

    def is_method?(name)

      name.start_with?('find_') ? true : false

    end

end

method_missing_spec.rb

require 'simplecov'
SimpleCov.start

require_relative 'method_missing'

describe 'Verify the method_missing on ruby meta programming' do

  before :each do
    @obj = MyClass.new
  end

  it 'Check methods that starts with "find_by_something" exists in MyClass' do
    method_exists = @obj.respond_to? 'find_by_name'
    expect(method_exists).to be true
  end

  it 'Check methods that not starts with "find_by_something" exists in MyClass' do
    method_exists = @obj.respond_to? 'test'
    expect(method_exists).to be false
  end

  it 'Raise a exception if call a method that not has find_by prefix' do
    expect { @obj.test }.to raise_error(NoMethodError)
  end

  it 'Call a method with prefix "find_by" from a obj of MyClass' do
    expect { @obj.find_by_value 'foo' }.not_to raise_error
  end

end