cti_schema_change.rb

require "logger"
require "sequel"

DB = Sequel.connect ENV["DATABASE_URL"], test: true, encoding: "utf-8"
# DB.logger = Logger.new($stdout)

require "sequel/plugins/class_table_inheritance"
module Sequel
  module Plugins
    module ClassTableInheritance
      def self.configure(model, opts = OPTS)
        SingleTableInheritance.configure model, opts[:key], opts

        model.instance_exec do
          @cti_models = [self]
          @cti_tables = [table_name]
          @cti_instance_dataset = @instance_dataset
          @cti_table_columns = columns
          @cti_table_map = opts[:table_map] || {}
          @cti_alias = opts[:alias] || @dataset.first_source.to_s.to_sym
          @cti_ignore_subclass_columns = opts[:ignore_subclass_columns] || []
        end
      end
    end
  end
end

Sequel.extension :migration

Sequel.migration do
  up do
    DB.create_schema :charts
    DB.run "SET search_path TO #{DB.literal("charts")};"

    create_table :artists do
      primary_key :id
      column :name, :text
    end

    create_table :albums do
      primary_key :id
      column :type, :text
      column :title, :text
      foreign_key :artist_id, :artists
    end

    create_table :sold_albums do
      column :id, :integer
      foreign_key [:id], :albums
    end
  end
end.apply(DB, :up)

DB.run "SET search_path TO #{DB.literal("public")};"

class Artist < Sequel::Model(Sequel[:charts][:artists])
  one_to_many :sold_albums
end

class Album < Sequel::Model(Sequel[:charts][:albums])
  plugin :class_table_inheritance,
    key: :type,
    table_map: {
      SoldAlbum: Sequel[:charts][:sold_albums]
    }

  many_to_one :artist
end

class SoldAlbum < Album
end

artist = Artist.create \
  name: "Artist"

album = SoldAlbum.create \
  title: "Album",
  artist_id: artist.id

puts artist.sold_albums_dataset.sql
puts artist.sold_albums