Using dataset erased all data from all tables in the test db. In my case, working with a legacy database that only needs to be read from, this is a Bad Thing. This monkey patch at the bottom of spec_helper.rb helps:

module Dataset
  module Database # :nodoc:
    
    # Provides Dataset a way to clear, dump and load databases.
    class Base
      include FileUtils
      
      def clear
        connection = ActiveRecord::Base.connection
        ActiveRecord::Base.silence do
          connection.tables.each do |table_name|
            unless %w{table_to_keep another_table even_another_table}.include? (table_name)
              puts "Emptying #{table_name}"
              connection.delete "DELETE FROM #{connection.quote_table_name(table_name)}",
                "Dataset::Database#clear" unless table_name == ActiveRecord::Migrator.schema_migrations_table_name
            else
              puts "Skipping #{table_name}"
            end
          end
        end
      end
    end
  end
end

It would be great to have a setting in dataset that lets me specify either which tables NOT to empty and/or which tables TO empty.