define_method: How to dynamically create methods with arguments

Question

I want to create a bunch of methods for a find_by feature. I don\'t want to write the same thing over and over again so I want to use metaprogramming.

Say I want to

Answer 1

When you do this: define_method("self.find_by_#{attribute}")

that is incorrect. The argument to define_method is a symbol with a single word.

Let me show you some correct code, hopefully this will be clear:

class MyClass < ActiveRecord::Base
  ["name", "brand"].each do |attribute|
    define_method(:"find_by_#{attribute}") do |attr_|
      first(attribute.to_sym => attr_)
    end
  end
end

This will produce class methods for find_by_brand and find_by_name.

Note that if you're looking into metaprogramming, this is a good use-case for method_missing. here's a tutorial to use method_missing to implement the same functionality you're going for (find_by_<x>)

Answer 2

It if you read the examples here http://apidock.com/ruby/Module/define_method you will find this one:

define_method(:my_method) do |foo, bar| # or even |*args|
  # do something
end

is the same as

def my_method(foo, bar)
   # do something
end

Answer 3

If I understand your question correctly, you want something like this:

class Product
  class << self
    [:name, :brand].each do |attribute|
      define_method :"find_by_#{attribute}" do |value|
        all.find {|prod| prod.public_send(attribute) == value }
      end
    end
  end
end

(I'm assuming that the all method returns an Enumerable.)

The above is more-or-less equivalent to defining two class methods like this:

class Product
  def self.find_by_name(value)
    all.find {|prod| prod.name == value }
  end

  def self.find_by_brand(value)
    all.find {|prod| prod.brand == value }
  end
end