When we first started Dash of Agile there were a couple of options for getting data from Pivotal Tracker to a rails app, but there were some issues with each of them. So, we created a brand-new tracker_api gem that does things right, and we've made it open source so that everyone can use it.
Our new Ruby wrapper for the Pivotal Tracker v5 API is thread-safe and takes advantage of Pivotal Tracker’s API fields parameter to allow eager loading of nested resources by fetching all data in a single request. The gem can exclude unnecessary fields, which reduces response sizes even further. It leverages Virtus to map JSON, and type-casts the values, to Ruby objects.