Hello, currently there's no way to set :default value for the :foreign_key attribute, and sometimes it may be useful.

1. Usage
   class Post
   belongs_to :user, default: lambda {User.current.id}

   end

1. Implementation
   Mongoid::Relations::Metadata.class_eval do
   def foreign_key_default_with_options
   self[:default] || foreign_key_default_without_options
   end
   alias_method_chain :foreign_key_default, :options
   end