Summary: don't use Single Table Inheritance (STI), use separate tables instead.
Rails makes it possible to have multiple models stored in the same table and map
these rows to the correct models using a type column. This can be used to for
example store two different types of SSH keys in the same table.
While tempting to use one should avoid this at all costs for the same reasons as outlined in the document "Polymorphic Associations".
The solution is very simple: just use a separate table for every type you'd
otherwise store in the same table. For example, instead of having a keys table
with type set to either Key or DeployKey you'd have two separate tables:
keys and deploy_keys.