It's generally bad practice to put constants in configs, for example to prevent load-order errors.

The field parameter type should allow a symbol arg and deprecate usage of constants for Mongoid 8.0.

field :first_name, type: :string   # new format
field :last_name, type: String     # deprecated

There are some gems that use custom types, one that comes to mind is Mongoid::EncryptedString. We should provide an API to register new symbol values as aliases to those types, e.g.:

Mongoid.register_type :encrypted_string, Mongoid::EncryptedString

As long as the class defines `mongoize` and `demongoize` it should work.