["name", "brand"].each do |attribute|
    define_method("self.find_by_#{attribute}") do |attr_|
      all.each do |prod|
        return prod if prod.attr_ == attr_
      end
    end
  end

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

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

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

def my_method(foo, bar)
   # do something
end

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