According to the ActiveRecord docs, even when :autosave is not present on an association, saving a parent model saves its associated child models as well.

What :autosave does, is to save them in the same transaction as the parent model:

> Note that :autosave => false is not same as not declaring :autosave. When the :autosave option is not present new associations are saved.

So I think even when :autosave => true is absent, Mongoid should still save the children. However when :autosave => false is specified, the children should not be automatically saved.

As MongoDB does not support multi-collection transactions at the moment, :autosave => true should be identical to no :autosave at all.