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The Ruby Subscript Operator

January 9, 2023

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Your Teacher

Chris Oliver

Hi, I'm Chris. I'm the creator of GoRails, Hatchbox.io and Jumpstart. I spend my time creating tutorials and tools to help Ruby on Rails developers build apps better and faster.

About This Episode

Rails uses the Ruby subscript operator to implement versioning with ActiveRecord Migrations. We'll learn how this works and implement it from scratch to see how it all ties together

Notes

Resources

module ActiveRecord
  class Migration

    # Receives a version of ActiveRecord as a float
    # Return a Class
    def self.[](version)
      class_name = "V#{version.to_s.gsub(".", "_")}" # => "V7_0"
      ActiveRecord.const_get(class_name)
    end

    def add_column(table_name, column_name, column_type)
      puts "Adding #{column_name} to #{table_name} table"
    end
  end

  class V7_0 < Migration
    def add_column(table_name, column_name, column_type)
      puts "Adding #{column_name} to #{table_name} table with old version 7.0"
    end
  end

  class V8_0 < Migration
  end
end

class AddEmailToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :email, :string
  end
end

class AddNameToUsers < ActiveRecord::Migration[8.0]
  def change
    add_column :users, :name, :string
  end
end

AddEmailToUsers.new.change
AddNameToUsers.new.change