[RoR] Validate length of relationship

gistfile1.rb

$ rails console
Loading development environment (Rails 3.1.1)
ruby-1.9.2-p290 :001 > c = Client.create
  SQL (16.3ms)  INSERT INTO "clients" ("name") VALUES (?)  [["name", nil]]
 => #<Client id: 11, name: nil> 
ruby-1.9.2-p290 :002 > 3.times do c.orders.create end
  Client Load (0.2ms)  SELECT "clients".* FROM "clients" WHERE "clients"."id" = 11 LIMIT 1
  Order Load (0.1ms)  SELECT "orders".* FROM "orders" WHERE "orders"."client_id" = 11
  SQL (0.4ms)  INSERT INTO "orders" ("client_id") VALUES (?)  [["client_id", 11]]
  Client Load (0.2ms)  SELECT "clients".* FROM "clients" WHERE "clients"."id" = 11 LIMIT 1
  Order Load (0.1ms)  SELECT "orders".* FROM "orders" WHERE "orders"."client_id" = 11
  SQL (0.3ms)  INSERT INTO "orders" ("client_id") VALUES (?)  [["client_id", 11]]
  Client Load (0.1ms)  SELECT "clients".* FROM "clients" WHERE "clients"."id" = 11 LIMIT 1
  Order Load (0.1ms)  SELECT "orders".* FROM "orders" WHERE "orders"."client_id" = 11
  SQL (0.2ms)  INSERT INTO "orders" ("client_id") VALUES (?)  [["client_id", 11]]
 => 3 
ruby-1.9.2-p290 :003 > c.valid?
 => true 
ruby-1.9.2-p290 :004 > c2 = Client.create
  SQL (0.6ms)  INSERT INTO "clients" ("name") VALUES (?)  [["name", nil]]
 => #<Client id: 12, name: nil> 
ruby-1.9.2-p290 :005 > 4.times do c2.orders.create end
  Client Load (0.2ms)  SELECT "clients".* FROM "clients" WHERE "clients"."id" = 12 LIMIT 1
  Order Load (0.1ms)  SELECT "orders".* FROM "orders" WHERE "orders"."client_id" = 12
  SQL (0.3ms)  INSERT INTO "orders" ("client_id") VALUES (?)  [["client_id", 12]]
  Client Load (0.2ms)  SELECT "clients".* FROM "clients" WHERE "clients"."id" = 12 LIMIT 1
  Order Load (0.1ms)  SELECT "orders".* FROM "orders" WHERE "orders"."client_id" = 12
  SQL (0.2ms)  INSERT INTO "orders" ("client_id") VALUES (?)  [["client_id", 12]]
  Client Load (0.1ms)  SELECT "clients".* FROM "clients" WHERE "clients"."id" = 12 LIMIT 1
  Order Load (0.1ms)  SELECT "orders".* FROM "orders" WHERE "orders"."client_id" = 12
  SQL (0.4ms)  INSERT INTO "orders" ("client_id") VALUES (?)  [["client_id", 12]]
  Client Load (0.2ms)  SELECT "clients".* FROM "clients" WHERE "clients"."id" = 12 LIMIT 1
  Order Load (0.1ms)  SELECT "orders".* FROM "orders" WHERE "orders"."client_id" = 12
  SQL (0.3ms)  INSERT INTO "orders" ("client_id") VALUES (?)  [["client_id", 12]]
 => 4 
ruby-1.9.2-p290 :006 > c2.valid?
 => false 
ruby-1.9.2-p290 :007 > c2.orders.length
 => 4 
ruby-1.9.2-p290 :008 > c2.orders.count
   (0.3ms)  SELECT COUNT(*) FROM "orders" WHERE "orders"."client_id" = 12
 => 4