Ruby - define_method, method_missing, and instance_eval

Ruby’s define_method, method_missing, and instance_eval are always mysterious to new starter. I put effort to understand what exactly they are. Here are the notes of what I’ve got.

Define_method

define_method lets you create methods using a method rather than the language builtin def. One major benefit of this is that you can reduce the duplication inherent methods with similar definitions. For example, the following methods all pull data out of an internal hash:

# Without define_method:
def user;  @data[:user];  end
def email; @data[:email]; end
def food;  @data[:food];  end

With define_method, we can iterate over each method name and reduce the duplication like so:

# With define_method:
%w(user email food).each do |meth|
  define_method(meth) { @data[meth.to_sym] }
end

Very easy to maintain.

Method_missng

method_missing, it?s the feature that puts the magic in Rails?s find_by_* methods. Defining all these find_by_* methods by hand is nearly impossible as there?s a large number of combinations and they?re based on columns in the database.

Example

class MyClass
  def foo(bar)
    17
  end
 
  def do_something(what, ntimes)
    puts "Something happened!"
  end
end

class Tracer
  def initialize(obj)
    @obj = obj
  end
  def method_missing(method_name, *args)
    puts "Called #{method_name} with args: #{args * ', '}"
    @obj.send(method_name,*args)
  end
end

Call the method

myclass = MyClass.new
wrapped_class = Tracer.new(myclass)
wrapped_class.do_something("blah", 5)
wrapped_class.foo(89)

Called do_something with args: blah, 5
Something happened!
Called foo with args: 89
17