gistfile1.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

p DB.tables #=> [:posts, :tags, :taggings]

Post.drop_table
p DB.tables #=> [:tags, :taggings]

Tag.drop_table
p DB.tables #=> [:taggings]

Tagging.drop_table
p DB.tables #=> []

Post.create_table
Tag.create_table
Tagging.create_table
p DB.tables #=> [:posts, :tags, :taggings]