Rails Metaprogramming: How to add instance methods at runtime?

Question

I'm defining my own AR class in Rails that will include dynamically created instance methods for user fields 0-9. The user fields are not stored in the db directly, they'll be serialized together since they'll be used infrequently. Is the following the best way to do this? Alternatives?

Where should the start up code for adding the methods be called from?

class Info < ActiveRecord::Base

end

# called from an init file to add the instance methods
parts = []
(0..9).each do |i|
   parts.push "def user_field_#{i}"     # def user_field_0
   parts.push   "get_user_fields && @user_fields[#{i}]"
   parts.push "end"
end

Info.class_eval parts.join

Marc-André Lafortune · Accepted Answer · 2010-03-29 18:49:01Z

11

One nice way, especially if you might have more than 0..9 user fields, would be to use method_missing:

class Info
  USER_FIELD_METHOD = /^user_field_(\n+)$/
  def method_missing(method, *arg)
    return super unless method =~ USER_FIELD_METHOD
    i = Regexp.last_match[1].to_i
    get_user_fields && @user_fields[i]
  end

  # Useful in 1.9.2, or with backports gem:
  def respond_to_missing?(method, private)  
    super || method =~ USER_FIELD_METHOD
  end
end

If you prefer to define methods:

10.times do |i|
  Info.class_eval do
    define_method :"user_field_#{i}" do
      get_user_fields && @user_fields[i]
    end
  end
end

answered Mar 29, 2010 at 18:49

Marc-André Lafortune

79.8k17 gold badges172 silver badges167 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Larry K Over a year ago

Thank you. Why will the [i] be eval'd at method definition rather than method execute time in your second example?

Marc-André Lafortune Over a year ago

The [i] will work as desired because the method definition is a block, and that in Ruby, blocks are closures. HTH

Andrew Grimm · Accepted Answer · 2011-03-27 22:55:15Z

4

Using method_missing is very difficult to maintain and unnecessary. The other alternative using define_method is better but leads to poorly performing code. The following 1 liner is all you need:

class Info
end

Info.class_eval 10.times.inject("") {|s,i| s += <<END}
  def user_field_#{i}
    puts "in user_field_#{i}"
  end
END

puts Info.new.user_field_4

edited Mar 27, 2011 at 22:55

Andrew Grimm

82.3k58 gold badges210 silver badges355 bronze badges

answered Mar 27, 2011 at 7:13

Wes

6,6053 gold badges24 silver badges26 bronze badges

Collectives™ on Stack Overflow

Rails Metaprogramming: How to add instance methods at runtime?

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related