Module method returns NoMethodError

Question

I'm trying to create a method inside my module that returns a hash with the given parameters.

module MyModule

  def self.my_method(*search_params)
    # do something...

    end
    self
  end
end

I've seen a lot of similar questions about this, where the issue is because I'm defining an instance method, but I called self in the definition and continue receiving the error, leading me to believe it might be something else.

@example = { :name => 'John', :quote => 'Great fun!', :rank => 5 }
@example.my_method(:name => 'John') 

NoMethodError: undefined method `my_method` (NoMethodError)

`@example` is an instance of a hash, do you want to add `my_method` to the hash class? — Anthony, Apr 27 '18 at 20:17
`@example` is a `Hash` and you are trying to call `MyModule::my_method` on it this will not work. Additionally `my_method` is a method specifically of that module due to the `self` declaration. There are other issues as well and I hate to be that guy but maybe read up on `Module`s a bit http://ruby-for-beginners.rubymonstas.org/advanced/modules.html — engineersmnky, Apr 27 '18 at 20:19

lacostenycoder · Accepted Answer · 2018-04-28T07:13:38.537

We can't understand what your method is trying to do because the logic doesn't make sense but here's how you add your method to the Hash class.

module MyModule
  def my_method(*search_params)
    puts "search params: #{search_params}"
  end
end


class Hash
  include MyModule
end

@example = { :name => 'John', :quote => 'Great fun!', :rank => 5 }
@example.my_method(:name => 'John')

#=>search params: [{:name=>"John"}]

However this is called "monkey patching" which is not recommended. It would probably be better to use inheritance

module MyModule
  def monkey(*search_params)
    puts "search params: #{search_params}"
  end
end


class MonkeyHash < Hash
  include MyModule
end

@example = MonkeyHash.new(:name => 'John', :quote => 'Great fun!', :rank => 5)
@example.monkey(:name => 'John')

@example = { :name => 'John', :quote => 'Great fun!', :rank => 5 }

begin
  @example.monkey(:name => 'John')
rescue NoMethodError => e
  puts "Calling @exmaple.my_method raiesed: "
  puts e
  puts "@example is an instance of #{@example.class}. You may want to use MonkeyHash"
  puts "which includes the instance method 'monkey'"
end

Or you could define a singleton method

puts "let's try it with a singleton method\n\n"

@singleton_example = { :name => 'John', :quote => 'Great fun!', :rank => 5 }

@singleton_example.define_singleton_method(:monkey) do |*search_params|
  puts "search params: #{search_params}"
end

puts "now @example has the monkey method see: \n"
@singleton_example.monkey(:name => 'John')
puts "@singleton_example is still an instance of #{@singleton_example.class}"

Okay, I think I get it. So as long as I'm creating a module that functions on a certain class, I have to state the inclusion, right? — Dylan Frye, Apr 27 '18 at 23:03
Yes, although are you sure you really want to modify the `Hash` class? This is called [monkey-patching](https://stackoverflow.com/questions/394144/what-does-monkey-patching-exactly-mean-in-ruby?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa). — Daryll Santos, Apr 28 '18 at 02:21
@DaryllSantos answer updated to deter monkey patching Hash class. — lacostenycoder, May 04 '18 at 17:31
Ah okay. I think I would actually just create a class to create the Person object with a hash in the constructor, just so I don't inherit from `Hash`. (Check out Primitive Obsession). — Daryll Santos, May 04 '18 at 19:21

score 0 · Answer 2 · answered Apr 28 '18 at 14:08

0

@example is a instance of a hash, while the module isn't included in the Hash class at all. You would need to add it to the Hash class first.

class Hash
  include MyModule
end

I would recommend you read the basics of Ruby classes and modules.

answered Apr 28 '18 at 14:08

thesimpledev

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Module method returns NoMethodError

2 Answers2