gistfile1.rb

require 'dm-core'
require 'dm-types'
require 'dm-migrations'
require 'dm-validations'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup :default, "sqlite::memory::"

class Person
  include DataMapper::Resource

  property :id, Serial
end

class Loan
  include DataMapper::Resource

  property :id, UUID, :key => true, :default => lambda { |r, p| UUIDTools::UUID.random_create }
end

class Ticket
  include DataMapper::Resource

  property :id, Serial

  property :loan_id, UUID, :unique => :loaner

  belongs_to :loaner, "Person"
  belongs_to :loanee, "Person"
  belongs_to :loan

  validates_with_block { loaner != loanee }
end

DataMapper.finalize.auto_migrate!

loaner = Person.create
loanee = Person.create

loan = Loan.create

t1 = Ticket.create(:loaner => loaner, :loanee => loanee, :loan => loan)

t2 = Ticket.new(:loaner => loaner, :loanee => loanee, :loan => loan)

puts t2.valid?
puts t2.errors.inspect

gistfile2.txt

 ~ (0.000069) SELECT sqlite_version(*)
 ~ (0.170509) DROP TABLE IF EXISTS "people"
 ~ (0.000036) PRAGMA table_info("people")
 ~ (0.157415) CREATE TABLE "people" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT)
 ~ (0.157829) DROP TABLE IF EXISTS "loans"
 ~ (0.000031) PRAGMA table_info("loans")
 ~ (0.174376) CREATE TABLE "loans" ("id" VARCHAR(36) NOT NULL, PRIMARY KEY("id"))
 ~ (0.182974) DROP TABLE IF EXISTS "tickets"
 ~ (0.000032) PRAGMA table_info("tickets")
 ~ (0.174169) CREATE TABLE "tickets" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "loan_id" VARCHAR(36), "loaner_id" INTEGER NOT NULL, "loanee_id" INTEGER NOT NULL)
 ~ (0.166520) CREATE INDEX "index_tickets_loaner" ON "tickets" ("loaner_id")
 ~ (0.166484) CREATE INDEX "index_tickets_loanee" ON "tickets" ("loanee_id")
 ~ (0.208128) CREATE UNIQUE INDEX "unique_tickets_loaner" ON "tickets" ("loan_id")
 ~ (0.315982) INSERT INTO "people" DEFAULT VALUES
 ~ (0.249397) INSERT INTO "people" DEFAULT VALUES
 ~ (0.140535) INSERT INTO "loans" ("id") VALUES ('724deb54-72de-4cb9-a8eb-ff23d93a39d4')
 ~ (0.000077) SELECT "id" FROM "tickets" WHERE ("loan_id" = '724deb54-72de-4cb9-a8eb-ff23d93a39d4' AND "loaner_id" = 1) ORDER BY "id" LIMIT 1
 ~ (0.146969) INSERT INTO "tickets" ("loan_id", "loaner_id", "loanee_id") VALUES ('724deb54-72de-4cb9-a8eb-ff23d93a39d4', 1, 2)
 ~ (0.000077) SELECT "id" FROM "tickets" WHERE ("loan_id" = '724deb54-72de-4cb9-a8eb-ff23d93a39d4' AND "loaner_id" = 1) ORDER BY "id" LIMIT 1
false
#<DataMapper::Validations::ValidationErrors:0x95d86e0 @resource=#<Ticket @id=nil @loan_id=#<UUID:0x4afb456 UUID:724deb54-72de-4cb9-a8eb-ff23d93a39d4> @loaner_id=1 @loanee_id=2>, @errors={:loan_id=>["Loan is already taken"]}>