This gem allows you to use sharded databases with ActiveRecord. This also provides a interface for replication and for running migrations with multiples shards.

DbCharmer is a Rails plugin (and gem) that could be used to manage AR model connections, implement master/slave query schemes, sharding and other magic features many high-scale applications need.

ActiveRecord connection proxy for master/slave connections

ActiveRecord sharding extension

Connection proxy for ActiveRecord for single master / multiple slave database deployments

Sharding and replication support for ActiveRecord

Horizontal scaling for your Rails or Merb app. Built on ActiveRecord.