Apr172010

Metaprogramming in Rails

Bagwan Pankaj
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Meta-programming means your programming language is writing code for you. And for sure Ruby is a great language having all cool features.Rails being a great ruby framework encashes all features of ruby. Rails itself uses meta-programming every where.

For example in ActiveRecord, we have dynamic method on attributes name like

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find_by_something(some_possible_values)
find_all_by_something(spv)
find_by_something_and_otherthing(some_value, other_value)

In this topic we will see how we can meta-program in Rails. Using Meta Programming one can reduce LOC to dramatic level and it also reduce headache in maintaining code base.

Here I am not going into details for class_eval and instanse_eval because there are many blog post out there. But I do give some examples.

Consider following:

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class Testt
 
end
 
Testt.class_eval do
  def foo
    "I'll be available as instance method"
  end
 
  def self.foo
    "I'll be available as class method"
  end
end
 
Testt.new().foo
=> I'll be available as instance method
Testt.foo
=>I  be available as class method

Let me explain that class_eval evaluates a string or code block in context of class or module it is called upon.

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Testt.instance_eval do
  def foo
    "I'll be available as class method"
  end
 
  def self.bar
    "I'll be available as class method"
  end
end
 
Testt.foo
=>I'll be available as class method
Testt.bar
=>I'll be available as class method

instance_eval can evaluate a string or a code block in the context of the receiver.

The third way to define a method in class or module is define_method.

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class Testt
  define_method :foo do
    "welcome"
  end
end
Testt.new().foo
=> welcome
 
module Hello
  define_method :foo do |name|
    "welcome #{name}"
  end
end
 
include Hello
foo "Dave"
=> welcome Dave

Now we can spice our project with metaprogramming. See how?
Suppose in our project we had a model to maintain users

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class User < ActiveRecord::Base
  #user has one role so association is
  has_one :role
end

We also have a Role model that belongs to user. Schema for role is suppose

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Role(id: integer, name: string, user_id: integer)

At the initiation of project the role known were admin and user. So I have defined two method in User model named as is_admin? and is_user?. But after some time we need to add some other roles as we need to define same function for them as well.

So to get rid of this we flavoured our project with bits of metaprogramming. See here how

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class User < ActiveRecord::Base
  #user has one role so association is
  has_one :role
  #First time when there is no method found; this method will catch that
  def self.method_missing(method_sym, *params)
    #Here we match the pattern of method and see if we can process it or not
    if method_sym.to_s =~ /^is_(.*)\?$/
      function = $1
 
      class_eval <<-END
	def method_sym.to_s
	  "#{role.name == function.to_s.upcase}"
        end
      END
      #Let's register this method with user class when it is invoked first time.
	send(method_sym)
    else
      super
    end
  end
 
  #Let this class respond for these methods
  def self.respond_to?(method_sym, *args, &amp;block)
    method_sym.to_s =~ /^is_(.*)\?$/ ? true : super
  end
end

Posted in Rails

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