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Rails 5 deprecates alias_method_chain

on August 21, 2016
This blog is part of our Rails 5 series.

Rails 5 has deprecated usage of alias_method_chain in favor of Ruby's built-in method Module#prepend.

What is alias_method_chain and when to use it

A lot of good articles have been written by some very smart people on the topic of "alias_method_chain". So we will not be attempting to describe it here.

Ernier Miller wrote When to use alias_method_chain more than five years ago but it is still worth a read.

Using Module#prepend to solve the problem

Ruby 2.0 introduced Module#prepend which allows us to insert a module before the class in the class ancestor hierarchy.

Let's try to solve the same problem using Module#prepend.

1module Flanderizer
2  def hello
3    "#{super}-diddly"
4  end
5end
6
7class Person
8  def hello
9    "Hello"
10  end
11end
12
13# In ruby 2.0
14Person.send(:prepend, Flanderizer)
15
16# In ruby 2.1
17Person.prepend(Flanderizer)
18
19flanders = Person.new
20puts flanders.hello #=> "Hello-diddly"

Now we are back to being nice to our neighbor which should make Ernie happy.

Let's see what the ancestors chain looks like.

1flanders.class.ancestors # => [Flanderizer, Person, Object, Kernel]

In Ruby 2.1 both Module#include and Module#prepend became a public method. In the above example we have shown both Ruby 2.0 and Ruby 2.1 versions.


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