Polymorphism/Polymorphic Associations

Polymorphism / Polymorphic Associations

Polymorphism

Polymorphism = Send the same message to different objects and get different results.

Inheritance

class GenericParser
  def parse
    raise NotImplementedError, 'No method'
  end
end

# inheritance 1
class JsonParser < GenericParser
  def parse
    puts 'JsonParser instance received a message.'
  end
end

# inheritance 2
class XmlParser < GenericParser
  def parse
    puts 'XmlParser instance received a message.'
  end
end

puts 'Using the XmlParser'
xml_parser = XmlParser.new
xml_parser.parse

puts 'Using the JsonParser'
json_parser = JsonParser.new
json_parser.parse

# Using the XmlParser
# XmlParser instance received a message.
# Using the JsonParser
# JsonParser instance received a message.

Duck Typing

class XmlParser
  def parse
    puts 'XmlParser!!!'
  end
end

class JsonParser
  def parse
    puts 'JsonParser!!!'
  end
end

class GenericParser
  def parse(parser)
    parser.parse
  end
end

gene_parser = GenericParser.new
puts 'Using the XmlParser'
gene_parser.parse(XmlParser.new)

puts 'Using the JsonParser'
gene_parser.parse(JsonParser.new)

# Using the XmlParser
# XmlParser!!!
# Using the JsonParser
# JsonParser!!!

Decorator

class Parser
  def parse
    puts 'Parser!!!'
  end
end

class XmlParser
  def initialize(parser)
    @parser = parser
  end

  def parse
      puts 'XmlParser'
    @parser.parse
  end
end

class JsonParser
  def initialize(parser)
    @parser = parser
  end

  def parse
    puts 'JsonParser'
    @parser.parse
  end
end

puts 'Using the XmlParser'
parser = Parser.new
XmlParser.new(parser).parse

puts 'Using the JsonParser'
JsonParser.new(parser).parse

puts 'Using both Parsers!'
JsonParser.new(XmlParser.new(parser)).parse

# Using the XmlParser
# XmlParser
# Parser!!!
# Using the JsonParser
# JsonParser
# Parser!!!
# Using both Parsers!
# JsonParser
# XmlParser
# Parser!!!

https://robots.thoughtbot.com/back-to-basics-polymorphism-and-ruby#inheritance

Polymorphic Associations

A model can belong to more than one other model, on a single association.

class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true
end

class Employee < ApplicationRecord
  has_many :pictures, as: :imageable
end

class Product < ApplicationRecord
  has_many :pictures, as: :imageable
end

# @employee.pictures.
# @product.pictures.

class CreatePictures < ActiveRecord::Migration[5.0]
  def change
    create_table :pictures do |t|
      t.string  :name
      t.integer :imageable_id
      t.string  :imageable_type
      t.timestamps
    end

    add_index :pictures, [:imageable_type, :imageable_id]
  end
end