```ruby
require 'mongoid'
require 'mongo_session_store/mongo_store_base'

module ActionDispatch
  module Session
    class MongoidStore < MongoStoreBase

      class Session
        include Mongoid::Document
        include Mongoid::Timestamps

        store_in :collection => MongoSessionStore.collection_name

        field :_id, :type => String

        field :data, :type => Moped::BSON::Binary, :default => Moped::BSON::Binary.new(:generic, Marshal.dump({}))

        attr_accessible :_id, :data
      end

      private
      def pack(data)
        Moped::BSON::Binary.new(:generic, Marshal.dump(data))
      end
    end
  end
end

MongoidStore = ActionDispatch::Session::MongoidStore
```

Fix MongoidStore for Mongoid 4 while remaining Mongoid 3 compatible.
user
```ruby
require 'mongoid'
require 'mongo_session_store/mongo_store_base'

module ActionDispatch
  module Session
    class MongoidStore < MongoStoreBase
      BINARY_CLASS = defined?(Moped::BSON::Binary) ? Moped::BSON::Binary : BSON::Binary

      class Session
        include Mongoid::Document
        include Mongoid::Timestamps

        store_in :collection => MongoSessionStore.collection_name

        field :_id, :type => String
        field :data, :type => BINARY_CLASS, :default => -> { marshaled_binary({}) }
        attr_accessible :_id, :data if respond_to?(:attr_accessible)

        def marshaled_binary(data)
          self.class.marshaled_binary(data)
        end

        def self.marshaled_binary(data)
          if BINARY_CLASS.to_s == 'BSON::Binary'
            BSON::Binary.new(Marshal.dump(data), :generic)
          else
            Moped::BSON::Binary.new(:generic, Marshal.dump(data))
          end
        end
      end

      private
      def pack(data)
        session_class.marshaled_binary(data)
      end

      def unpack(packed)
        return nil unless packed
        Marshal.load(extract_data(packed))
      end

      def extract_data(packed)
        if packed.class.to_s == 'BSON::Binary'
          packed.data
        else
          packed.to_s
        end
      end
    end
  end
end

MongoidStore = ActionDispatch::Session::MongoidStore
```
assistant