I found when writing a spec helper recently that I should have been destroying all models before each test like Rails would.

Thanks to the awesome DKubb from #datamapper on FreeNode for this tip that should have been obvious to me. In my spec/spec_helpers.rb file, I have added the following configuration option:

Spec::Runner.configure do |config|
  config.before(:all) do
    DataMapper::Model.descendants.each {|m| m.all.destroy }
  end
end

Make sure that you are working on your _test database of course. You may want to use :each instead of :all depending on your tests. Remember that :each is run before each example, :all for each example group.