Why does define_method not create a new class method in this example?

Question

I have this code:

module Helper
  def translates(*attributes)
    attributes.each do |attribute|
      define_method("find_by_#{attribute}") do |value|
        value
      end  
    end  
  end  
end  

class SomeClass
  extend Helper
  translates :foo
end

Now, in my opinion, the method SomeClass.find_by_foo should exist. But it doesn't. Do you know what am I doing wrong?

It exists but you defined it as an instance method SomeClass.new.find_by_foo(1) — astropanic, Jul 17 '12 at 13:52
Take a look at a similar question: http://stackoverflow.com/questions/752717/how-do-i-use-define-method-to-create-class-methods — Teoulas, Jul 17 '12 at 13:54

score 2 · Answer 1 · answered Jul 17 '12 at 13:54

2

You'll find that SomeClass.new.respond_to?(:find_by_foo) returns true. If you want to add the methods to the class side, use define_singleton_method.

answered Jul 17 '12 at 13:54

Frank Shearar

17,012
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score 1 · Accepted Answer · answered Jul 17 '12 at 13:57

1

Use the Eigenclass

You can define the method as a class method using the eigenclass. For example:

module Helper
  def translates(*attributes)
    attributes.each do |attribute|
      define_singleton_method("find_by_#{attribute}") do |value|
        value
      end
    end
  end
end

answered Jul 17 '12 at 13:57

Todd A. Jacobs

81,402
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Thanks. I tried to put `define_method` inside `instance_eval` but it didn't work. I didn't know about this one. – Janko Jul 17 '12 at 14:14

Why does define_method not create a new class method in this example?

2 Answers2

Use the Eigenclass