Extend return value of class instance method

Question

I have a class that have an instance method, that returns a hash. I can't change the code of that class directly, but I can extend it with modules. I need to add some new keys to the returning hash of the method. Something like this:

class Processor
  def process
    { a: 1 }
  end
end

module ProcessorCustom
  def process
    super.merge(b: 2) # Not works :(
  end
end

Processor.send :include, ProcessorCustom

processor = Processor.new
processor.process  # returns { a: 1 }, not { a: 1, b: 2 }

How can I do that? Thanks.

Answer 1

You could call prepend instead of include:

Processor.prepend(ProcessorCustom)

processor = Processor.new
processor.process
#=> {:a=>1, :b=>2}

prepend and include result in different ancestor order:

module A; end
module B; end
module C; end

B.ancestors     #=> [B]

B.include(C)
B.ancestors     #=> [B, C]

B.prepend(A)
B.ancestors     #=> [A, B, C]

Alternatives

Depending on your use-case, you could also extend a specific instance: (this doesn't affect other instances)

processor = Processor.new
processor.extend(ProcessorCustom)
processor.process
#=> {:a=>1, :b=>2}

Or use SimpleDelegator to implement a decorator pattern:

require 'delegate'
class ProcessorCustom < SimpleDelegator
  def process
    super.merge(b: 2)
  end
end

processor = ProcessorCustom.new(Processor.new)
processor.process  #=> {:a=>1, :b=>2}