Mongoid allows the classic ActiveRecord associations:
has_one / belongs_tohas_many / belongs_tohas_and_belongs_to_manyTo add an association (lets say the User has_many posts), you can add this to your User model file:
has_many :posts
and this to your Post model file:
belongs_to :user
This will add a user_id field in your Post model, add a user method to your Post class, and add a posts method to your User class.