This version includes two important changes:
Calling id on new instances
In previous versions, trying to access an instance's id would raise a MissingID error. The reason why raising that error is convenient is because it allows Ohm to fail as early as possible when you try to use an object that hasn't been saved yet. The error can arise from comparisons, from direct calls to id, and also from any call to methods that need object.key to return a proper value, as key in turn calls the id method. But it turns out that many form builders rely on the fact that sending the id message to most ORMs results in either a valid ID or nil, and here Ohm was the exception. By moving the check from id to key, we can keep most of the previous behavior and we can return nil when sending the id message to a new instance, thus making Ohm compatible with most form builders.
Internal encoding
When Ohm started using Lua internally for saving, updating and deleting objects, it used msgpack for serializing. Redis supports both msgpack and JSON, and we picked msgpack because it produces a more compact representation. At some point, the Ruby msgpack library and the internal msgpack support in Redis diverged, and we were forced to lock the dependency to a specific gem version. Recently, some people complained about encoding issues originating from msgpack inside Redis, and once they tried a modified Ohm that uses JSON instead of msgpack, all their issues disappeared. That's why we think it's time for removing msgpack altogether and use JSON instead.