You can tell Mongoid that certain attributes are read-only. This will allow documents to be created with these attributes, but changes to them will be ignored when using mass update methods such as update_attributes:
Individual assignment appears to be equally ignored:
The individual assignment should either raise ReadonlyAttribute instead of being ignored like that or this behavior should be documented.
Alternatively, if this is the desired behavior, adding a use case to documentation would help clarify when one should use read-only attributes.