Example of using polymorphism to replace switch statement

all.rb

# frozen_string_literal: true

module PercyDbMigrate
  module PrereqPolicy

    # A prereq policy that migrates both selected classes and
    # prerequesites.
    class All

      # Return the migration classes to run.
      #
      # @return [MigrationClasses]
      def migration_classes(selected_migration_classes)
        selected_migration_classes + selected_migration_classes.dependencies
      end

    end

  end

end

arguments.rb

      @prereq_policy_name = PrereqPolicy.default_name

      ...
        
        op.on("-p S", PrereqPolicy.names,
              "Prerequesite policy (default #{@prereq_policy})",
              "One of #{PrereqPolicy.names.join(", ")}",
             ) do |v|
          @prereq_policy_name = v
        end

main.rb

# Create a policy based on its name given in a command-line switch
@prereq_policy = PrereqPolicy[@args.prereq_policy_name]
...
# use the policy to select which migrations to run
@migration_classes = @prereq_policy.migration_classes(selected_migration_classes)

none.rb

# frozen_string_literal: true

require "active_support"

module PercyDbMigrate
  module PrereqPolicy

    # A prereq policy that migrates no prerequisites.
    class None

      # Return the migration classes to run.
      #
      # @return [MigrationClasses]
      def migration_classes(selected_migration_classes)
        selected_migration_classes
      end

    end

  end

end

only.rb

# frozen_string_literal: true

require "active_support"

module PercyDbMigrate
  module PrereqPolicy

    # A prereq policy that migrates only prerequisites.
    class Only

      # Return the migration classes to run.
      #
      # @return [MigrationClasses]
      def migration_classes(selected_migration_classes)
        selected_migration_classes.dependencies
      end

    end

  end

end

prereq_policy.rb

# frozen_string_literal: true

require "active_support"

module PercyDbMigrate

  # Namespace and factory for various prerequesite policies.
  #
  # A prerequest policy determines what tables to fetch when
  # migrations are being run.
  module PrereqPolicy

    extend ActiveSupport::Autoload

    autoload :All
    autoload :None
    autoload :Only

    # Map names to policies
    MAP = {
      "all" => All,
      "none" => None,
      "only" => Only,
    }
    private_constant :MAP

    # Return the name of the default policy
    def self.default_name
      "all"
    end

    # Return the names of all policies
    def self.names
      MAP.keys
    end

    # Return the named policy.
    def self.[](name)
      MAP.fetch(name).new
    end

    # Return the default policy.
    def self.default
      self[default_name]
    end

  end

end