title: Testing different database backends on a rails gem
body: <p>I need orientation implementing the tests of a gem I'd like to build. It's a small gem that adds a new class method to <code>ActiveRecord::Base</code>. This new method will execute some SQL on demmand.</p>
  
  <p>Trouble is, the SQL to be executed depends on the database backend. It's different for SQLite, Postgres and MySQL. The ruby method signature is the same for all database backends - only the SQL changes.</p>
  
  <p>So, how do I make the tests for this?</p>
  
  <p>To clarify: I know how to ask ActiveRecord "what database backend are you on?". What I need is a way to tell RSpec/Minitest/TestUnit "change the database backend to Postgres, and try all tests again. And then MySQL".</p>