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Method alias in ruby

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In ruby you can create aliases for a method and variable name. This can be helpful if you want to override the behavior of some method without changing the origin implementation of it. alias_method take a new_name as a copy name of the old_name and it has the following syntax.

alias_attribute (new_name, old_name)

A small example:

class Davi
  def capital
    puts "Karaz-a-Karak"
  end

  alias_method :orig_capital, :capital

  def capital
    puts "Karaz-a-Karak rebuild"
    orig_capital
  end
end

davi = Davi.new
davi.capital

# output
"Karaz-a-Karak rebuild"
"Karaz-a-Karak"

What is the difference between alias and alias_method

alias is more general than alias_method and can be used to create an alias for global variable, regular expression backreference (like $&) or an existing method. Class variables, local variables, instance variables and constants may not be aliased.

def khemri_city
  puts "Nehekhara"
end

alias :orig_khemri_city :khemri_city

def khemri_city
  puts "Nehekhara new"
end

khemri_city

# output
"Nehekhara"
"Nehekhara new"

alias_method must return must be called on a method. So alias is more general than alias_method.

Conclusion

It is possible with alias_method to reopen a class, override a method call and you can still use the original call. In order to maintain backward compatibility alias_method are used in plugins, extensions, deprecating variables. Alias_method can be used in Rails to define action with duplicated content and remove duplicated code.

Here the duplicated variant:

class UsersController < ApplicationController
  def home
    list
  end

  def find
    list
  end

  def search
    list
  end
end

The DRY variant:

class UsersController < ApplicationController
  def home
    list
  end

  alias_method :find, :home
  alias_method :search, :home
end

Further reading