dangerous polymorphic fun with primary key casts (nsfw)

castaway.rb

class CastIdsToText < ActiveRecord::Migration[6.0]
  def up
    add_cast :text, :uuid, function: :inout, context: :implicit
    add_cast :text, :bigint, function: :inout, context: :implicit
  end

  def down
    remove_cast :text, :uuid
    remove_cast :text, :bigint
  end

  private
    def add_cast(source_type, target_type, function: :binary, context: nil)
      cast_argument = "#{source_type} AS #{target_type}"

      method_argument = case function
      when :inout
        "WITH INOUT"
      when :binary, nil
        "WITHOUT FUNCTION"
      else
        "WITH FUNCTION #{function.to_s}"
      end

      context_argument = case context
      when :implicit
        "AS IMPLICIT"
      when :assignment
        "AS ASSIGNMENT"
      else
        context.to_s
      end

      connection.execute <<~DDL
        DO $$ BEGIN
          CREATE CAST (#{cast_argument}) #{method_argument} #{context_argument};
        EXCEPTION
          WHEN duplicate_object THEN null;
        END $$;
      DDL
    end

    def remove_cast(source_type, target_type)
      cast_argument = "#{source_type} AS #{target_type}"

      connection.execute <<~DDL
        DROP CAST IF EXISTS (#{cast_argument});
      DDL
    end
end