rails--rails/activerecord/test/models/vertex.rb

# This class models a vertex in a directed graph.
class Vertex < ActiveRecord::Base
  has_many :sink_edges, class_name: "Edge", foreign_key: "source_id"
  has_many :sinks, through: :sink_edges

  has_and_belongs_to_many :sources,
    class_name: "Vertex", join_table: "edges",
    foreign_key: "sink_id", association_foreign_key: "source_id"
end