If a document is fetched with limited fields which don't include a field with a default, and then updated, the field with a default is set.

That is:

class Thing
field :field, default: "just the default"
end

thing = Thing.new(field: "not default")
thing.save!
thing2 = Thing.where(_id: thing.id).only(:_id)
thing2.update
thing3 = Thing.find(thing.id)
thing3.field #=> "just the default"