If you're not familiar with Rails and ActiveRecord in general or with ActiveModel in particular then MongoMapper can be difficult to cope with at times. Even though the docs are fine and in detail some parts seem to be missing that are probably obvious for those coming from Rails. I'm not a Rails guy so I post my experience with many to many associations that might help one or the other save some precious debugging hours.
Take this code:
class User
include MongoMapper::Document
key :name, String, :required => true
key :email, String
key :post_ids, Array