If a field is a container object (Set or Array, for example), changes to this object are not recognized by Mongoid and are not persisted.
The value of the attribute must be reset for Mongoid to persist the new data.
This issue was originally filed for Set fields, https://jira.mongodb.org/browse/MONGOID-3743 is another instance of this issue for Array fields.
Not sure what's going on here but I cannot add to sets embedded in a mongoid document.
same result with default: Set.new
Here is an example with a string field: