one_to_one_update.rb

require "sequel"

DB = Sequel.sqlite

DB.create_table :albums do
  primary_key :id
  column :title, :text
end

DB.create_table :artists do
  primary_key :id
  foreign_key :album_id, :albums, null: false
  column :name, :text
end

DB.create_table :covers do
  primary_key :id
  foreign_key :album_id, :albums, null: false
  column :color, :text
end

class Album < Sequel::Model
  plugin :nested_attributes

  one_to_many :artists
  nested_attributes :artists

  one_to_one :cover
  nested_attributes :cover
end

class Artist < Sequel::Model
  many_to_one :album

  def validate
    puts "#{self} validate #{album}"
    super

    errors.add :name, "cannot be album name" if name == album.title
  end

  def after_save
    puts "#{self} after_save #{album}"
    album.title
  end
end

class Cover < Sequel::Model
  many_to_one :album

  def validate
    puts "#{self} validate #{album}"
    super

    errors.add :color, "cannot be album name" if color == album.title
  end
  
  def after_save
    puts "#{self} after_save #{album}"
    album.title
  end
end

puts "create Album + one_to_many Artist + one_to_one Cover"
Album.create \
  title: "Album 1",
  artists_attributes: [{
    name: "Bill"
  }],
  cover_attributes: {
    color: "Red"
  }

puts "create Album"
album = Album.create \
  title: "Album 2"

puts "update Album + one_to_many Artist"
album.update \
  artists_attributes: [{
    name: "Steve"
  }]

puts "update Album + one_to_one Cover"
album.update \
  cover_attributes: {
    color: "Blue"
  }