title: Active record Undefined method `eq'
body: <p>I've table tournaments with fields user_id,question_id,answer and I created key like</p>
  
  <pre><code>Primary key (user_id, question_id)
  </code></pre>
  
  <p>But when I want to change answer field like this:</p>
  
  <pre><code>t=Tournament.where('question_id=200').first
  t.answer=2
  t.save
  </code></pre>
  
  <p>I've got error:</p>
  
  <pre><code>NoMethodError: undefined method `eq' for nil:NilClass
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.0.4/lib/active_support/whiny_nil.rb:48:in `method_missing'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/persistence.rb:255:in `update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/locking/optimistic.rb:77:in `update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/attribute_methods/dirty.rb:68:in `update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/timestamp.rb:60:in `update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/callbacks.rb:285:in `block in update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.0.4/lib/active_support/callbacks.rb:413:in `_run_update_callbacks'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/callbacks.rb:285:in `update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/persistence.rb:246:in `create_or_update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/callbacks.rb:277:in `block in create_or_update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.0.4/lib/active_support/callbacks.rb:423:in `_run_save_callbacks'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/callbacks.rb:277:in `create_or_update'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/persistence.rb:56:in `save!'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/validations.rb:49:in `save!'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/attribute_methods/dirty.rb:30:in `save!'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/transactions.rb:245:in `block in save!'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/transactions.rb:292:in `block in with_transaction_returning_status'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/connection_adapters/abstract/database_statements.rb:139:in `transaction'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/transactions.rb:207:in `transaction'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/transactions.rb:290:in `with_transaction_returning_status'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/transactions.rb:245:in `save!'
      from (irb):4
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands/console.rb:44:in `start'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands/console.rb:8:in `start'
      from /home/marat/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.0.4/lib/rails/commands.rb:23:in `&lt;top (required)&gt;'
      from script/rails:6:in `require'
  </code></pre>
  
  <p>Can anyone help me?
  Thanks.</p>