Storing query cache in Thread.current breaks when multiple DBs are used

We use rails with multiple databases. Some models come from one database, and some from another. We use establish_connection to make sure the models are connected correctly. (Using the use_db plugin.)

Consider the following code from my unit tests:

def test_database
  User.cache do
    assert_equal "database1_test", User.connection.current_database, "db must be correct"
    assert_equal "database2_test", Product.connection.current_database, "db must be correct"
  end
end

This fails in 2.2 because the query cache is not stored on a per connection basis.

User.cache sets Thread.current['query_cache_enabled'] to true for the block.

Then because the query cache is stored in Thread.current['query_cache'] and because the two queries are the same (SELECT DATABASE() as db) the second call to connection.current_database returns the wrong result.

Perhaps this could be fixed by adding the database name to the key used, like this in active_record/connection_adapters/abstract/query_cache.rb:

      def query_cache
        Thread.current["query_cache_#{@config[:database]}"] ||= {}
      end

      def query_cache=(cache)
        Thread.current["query_cache_#{@config[:database]}"] = cache
      end

With this patch tests pass (and my real code now works).

This isn't quite perfect though - we have to add the ||= assignment to query_cache because calling User.cache only initialises the hash for the db associated with the User model.

Anyone have a better idea?