It seems that somewhere since https://github.com/mongoid/mongoid/commit/8c4794f25f177feff8e8b49fb42e1184b186ff56, the ability to do has_many :children has been broken. I noticed it as having a model that has_many :children and embeds_many :somethings breaks nested attributes pretty badly, see: https://github.com/mongoid/mongoid/issues/3144 ...still looking for a workaround without rewriting large chunks of an app.

On a similar subject, you can no longer have a model that embeds_many :fields and a Field model as this also causes breakages. I don't know if this is related but is also a common use-case.