This line's missing `data_source`. :-)

Also, without putting these configure calls in `before(:all) { }` blocks, you won't get the results you're looking for due to how RSpec evaluates spec examples. It will evaluate all of the describe blocks (context is an alias for describe) first before executing any examples, so any code not in a `before`, `after` or `it` block runs on load, meaning it'll configure with a `Perpetuity::Memory`, then with a `Perpetuity::MongoDB`, and _then_ execute the examples.

I've run into this at least a half-dozen times on various projects. :-)