title: Rails: "invalid byte sequence in UTF-8" exception when setting up a has_one association by UUID
body: <p>Environment: Ruby 1.9.2, Rails 3.0.1</p>
  
  <p>Two MySQL tables: 1. items, 2. vote_counts (every row represents the number of users who voted on an item)</p>
  
  <p>Since the tables are big and I need to shard them, I am using UUIDs. The uuid column is defined in both tables: <code>uuid</code> varbinary(16) NOT NULL.</p>
  
  <p>I defined a has_one association in item model: has_one :vote_count, :foreign_key => "uuid", :primary_key => "uuid"</p>
  
  <p>When I call item.vote_count, I am getting the following exception on some UUID values:
  "ArgumentError: invalid byte sequence in UTF-8".</p>
  
  <p>Of course the raw UUID is just a sequence of bytes (Encoding:ASCII-8BIT), but when ActiveRecord constructs a SQL query, it tries to interpret it as a UTF-8 string.</p>
  
  <p>How can I tell it to just pass the byte sequence to MySQL?</p>