gistfile1.rb

ruby-1.9.2-p290 :043 > SecureRandom.hex(8)
 => "4935b273ef3ff39b" 
ruby-1.9.2-p290 :044 > token = _
 => "4935b273ef3ff39b" 
ruby-1.9.2-p290 :045 > Cart.create(:token => token)
  SQL (0.7ms)  INSERT INTO "carts" ("created_at", "token", "updated_at") VALUES (?, ?, ?)  [["created_at", Mon, 17 Oct 2011 15:06:10 UTC +00:00], ["token", "4935b273ef3ff39b"], ["updated_at", Mon, 17 Oct 2011 15:06:10 UTC +00:00]]
 => #<Cart id: 4, token: "4935b273ef3ff39b", created_at: "2011-10-17 15:06:10", updated_at: "2011-10-17 15:06:10"> 
ruby-1.9.2-p290 :046 > Cart.where(:token => token)
  Cart Load (0.3ms)  SELECT "carts".* FROM "carts" WHERE "carts"."token" = '4935b273ef3ff39b'
 => [] 

this works:

ruby-1.9.2-p290 :058 > token = "another simple string"
 => "another simple string" 
ruby-1.9.2-p290 :059 > Cart.create(:token => token)
  SQL (1.1ms)  INSERT INTO "carts" ("created_at", "token", "updated_at") VALUES (?, ?, ?)  [["created_at", Mon, 17 Oct 2011 15:14:57 UTC +00:00], ["token", "another simple string"], ["updated_at", Mon, 17 Oct 2011 15:14:57 UTC +00:00]]
 => #<Cart id: 7, token: "another simple string", created_at: "2011-10-17 15:14:57", updated_at: "2011-10-17 15:14:57"> 
ruby-1.9.2-p290 :060 > Cart.where(:token => token)
  Cart Load (0.3ms)  SELECT "carts".* FROM "carts" WHERE "carts"."token" = 'another simple string'
 => [#<Cart id: 7, token: "another simple string", created_at: "2011-10-17 15:14:57", updated_at: "2011-10-17 15:14:57">] 
ruby-1.9.2-p290 :061 >

and this works too:

ruby-1.9.2-p290 :067 > token = Cart.find(4).token
  Cart Load (0.3ms)  SELECT "carts".* FROM "carts" WHERE "carts"."id" = ? LIMIT 1  [["id", 4]]
 => "4935b273ef3ff39b" 
ruby-1.9.2-p290 :068 > Cart.where("token like ?",token)
  Cart Load (0.2ms)  SELECT "carts".* FROM "carts" WHERE (token like '4935b273ef3ff39b')
 => [#<Cart id: 4, token: "4935b273ef3ff39b", created_at: "2011-10-17 15:06:10", updated_at: "2011-10-17 15:06:10">] 
ruby-1.9.2-p290 :069 > 

this was the problem:

ruby-1.9.2-p290 :069 > token.encoding
 => #<Encoding:ASCII-8BIT> 
ruby-1.9.2-p290 :070 > Cart.last.token.encoding
  Cart Load (0.2ms)  SELECT "carts".* FROM "carts" ORDER BY "carts"."id" DESC LIMIT 1
 => #<Encoding:UTF-8> 
ruby-1.9.2-p290 :071 >

This is the solution:

SecureRandom.hex(18).force_encoding(Cart.connection.encoding)
This is not required for mysql2, as it forces utf-8 by itself.