I do not understand a many_to_one <=> one_to_one model association - ruby

Question

how to say? I do not understand what the sequel documentation tries to tell me about associations in case of a two models linked over a foreign key in one model A being a primary key in the other in a may_to_one case.
I always thought: If it is many_to _one in one direction it has to be one_to_many in the other... but the sequel provides a confusing chapter meant to clarify the topic with in addition an example I cannot follow.
It says in
"Differences Between many_to_one and one_to_one"
If you want to setup a 1-1 relationship between two models, where the foreign > key in one table references the associated table directly, you have to use
many_to_one in one model, and one_to_one in the other model. How do
you know which to use in which model? The simplest way to remember is
that the model whose table has the foreign key uses many_to_one, and
the other model uses one_to_one"
And continues to provide this strange example:
# Database schema:
# artists albums
# :id <----\ :id
# :name \----- :artist_id
# :name
class Artist
one_to_one :album
end
class Album
many_to_one :artist
end
In albums I may find several rows pointing to same artist... why shouldn't the artist point back to all his/her albums?
The sequel docu is crazy hard to read in many cases but this chapter reads easy but makes no sense for me:(

Answer 1

Same issue for me.
require "logger"
require "sequel"
db = Sequel.connect "postgres://localhost/postgres", :logger => Logger.new(STDOUT)
db.drop_table :artists, :cascade => true if db.table_exists?(:artists)
db.create_table :artists do
primary_key :id
foreign_key :album_id, :albums
end
db.drop_table :albums, :cascade => true if db.table_exists?(:albums)
db.create_table :albums do
primary_key :id
foreign_key :artist_id, :artists
end
class Artist < Sequel::Model(db[:artists])
one_to_one :album
end
class Album < Sequel::Model(db[:albums])
one_to_one :artist
end
artist_1 = Artist.create
album_1 = Album.create
artist_1.update :album => album_1
album_1.reload
puts album_1.artist.nil?
artist_2 = Artist.create
album_2 = Album.create
album_2.update :artist => artist_2
artist_2.reload
puts artist_2.album.nil?
We can fix this example by replacing any of one_to_one into many_to_one.
class Album
many_to_one :artist
end
In this case artist.album_id won't be used.
class Artist
many_to_one :albums
end
In this case album.artist_id won't be used.
The problem is that method names one_to_one and many_to_one were selected by underlying sequel logic and they are not user friendly.
You can create user friendly aliases for these methods. I prefer just to use it with comments. For example:
db.create_table :artists do
primary_key :id
foreign_key :album_id, :albums
end
db.create_table :albums do
primary_key :id
end
class Artist < Sequel::Model(db[:artists])
many_to_one :album # I have album_id foreign key
end
class Album < Sequel::Model(db[:albums])
one_to_one :artist # I don't have artist_id foreign key
end