class Join < ActiveRecord::Base
  def Join.for *args
    desc = args.shift
    src, dst, *ignored = desc.to_a.flatten

    src_options = (args.shift || {}).to_options
    dst_options = (args.shift || {}).to_options

    const = "#{ src }_#{ dst }".camelize

    unless const_defined?(const)
      join_model = Class.new(Join)
      const_set const, join_model
      join_model.module_eval do
        belongs_to src.to_s.to_sym, src_options.reverse_merge(:foreign_key => :src)
        belongs_to dst.to_s.to_sym, dst_options.reverse_merge(:foreign_key => :dst)
      end
    end

    "::Join::#{ const }"
  end

  class Migration < ActiveRecord::Migration
    def self.up
      create_table :joins do |t|
        t.string :type  # sti
        t.integer :src
        t.integer :dst
        t.timestamps
      end

      add_index :joins, [:type]
      add_index :joins, [:src]
      add_index :joins, [:dst]
      add_index :joins, [:src, :dst], :unique => true
    end

    def self.down
      remove_index :joins, [:src, :dst]
      remove_index :joins, [:dst]
      remove_index :joins, [:src]
      remove_index :joins, [:type]

      drop_table :joins
    end
  end
end