match.rb

class Match < ActiveRecord::Base
  belongs_to :p1, class_name: 'Player'
  belongs_to :p2, class_name: 'Player'

  def find_by_player(player)
    where('p1_id = :player OR p2_id = :player', player: player.id)
  end

  def opponent_of(player)
    if p1 == player ? p2 : p1
  end
end

player.rb

class Player < ActiveRecord::Base
  def matches
    Match.find_by_player(self)
  end

  def opponents
    matches.collect { |m| m.opponent_of(self) }
  end
end

Schema

ActiveRecord::Schema.define(version: 20131006225214) do

  create_table "matches", force: true do |t|
    t.integer  "result"
    t.boolean  "checked_out", default: false
    t.boolean  "repeated",    default: false
    t.boolean  "planned",     default: true
    t.integer  "round"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.integer  "p1_id"
    t.integer  "p2_id"
  end

  create_table "players", force: true do |t|
    t.integer  "in_tournament_id"
    t.boolean  "byed",             default: false
    t.integer  "matches",          default: 0
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "tournaments", force: true do |t|
    t.boolean  "allow_repeat",      default: false
    t.boolean  "locked",            default: false
    t.integer  "number_players"
    t.integer  "rounds"
    t.integer  "matches_per_round"
    t.integer  "additional_rounds"
    t.integer  "round",             default: 0
    t.datetime "created_at"
    t.datetime "updated_at"
  end

end