CODEWITHSUNDEEP
rubymetaprogramming
Kostas Andrianos
Kostas Andrianos

Reputation: 1619

define_method: How to dynamically create methods with arguments

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 create a method for finding by name, accepting the name as an argument. How would I do it? I've used define_method in the past but I didn't have any arguments for the method to take. Here's my (bad) approach

["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

Any thoughts? Thanks in advance.

Upvotes: 46

Views: 43502

Answers (3)

max pleaner
max pleaner

Reputation: 26758

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>)

Upvotes: 4

Jordan Running
Jordan Running

Reputation: 106027

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

Upvotes: 58

Albin
Albin

Reputation: 3012

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

Upvotes: 12

Related Questions