sequel_association.rb

#!/usr/bin/ruby19

require 'sequel'

Sequel::Model.plugin :schema 

DB = Sequel.sqlite

class Post < Sequel::Model
  set_schema do
    primary_key :id
  end
  many_to_many :tags, :join_table => :taggings

  create_table unless table_exists?
end

class Tag < Sequel::Model
  set_schema do
    primary_key :id
  end
  many_to_many :posts, :join_table => :taggings

  create_table unless table_exists?
end

class Tagging < Sequel::Model
  set_schema do
    primary_key :id
    foreign_key :post_id, :table => :posts
    foreign_key :tag_id, :table => :tags
  end
  many_to_one :posts
  many_to_one :tags

  create_table unless table_exists?
end

post = Post.create
tag  = Tag.create
post.add_tag tag

p post.tags     #=> [#<Tag @values={:id=>1}>]
p tag.posts     #=> [#<Post @values={:id=>1}>]
p Tagging.all   #=> [#<Tagging @values={:id=>1, :post_id=>1, :tag_id=>1}>]
p Post.all      #=> [#<Post @values={:id=>1}>]
p Tag.all       #=> [#<Tag @values={:id=>1}>]