I've encountered some problems with MongoID. I have three models: require 'mongoid' class Configuration include Mongoid::Document belongs_to :user field :links, :type => Array field :root, :type => String field :objects, :type => Array field :categories, :type => Array has_many :entries end class TimeDim include Mongoid::Document field :day, :type => Integer field :month, :type => Integer field :year, :type => Integer field :day_of_week, :type => Integer field :minute, :type => Integer field :hour, :type => Integer has_many :entries end class Entry include Mongoid::Document belongs_to :configuration belongs_to :time_dim field :category, :type => String # any other dynamic fields end Creating documents for Configurations and TimeDims is successful. But when i've trying to execute following code: params = Hash.new params[:configuration] = config # an instance of Configuration from DB entry.each do |key, value| params[key.to_sym] = value # String end unless Entry.exists?(conditions: params) params[:time_dim] = self.generate_time_dim # an instance of TimeDim from DB params[:category] = self.detect_category(descr) # String Entry.new(params).save end ... i saw following output: /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/bson-1.6.1/lib/bson/bson_c.rb:24:in `serialize': Cannot serialize an object of class Configuration into BSON. (BSON::InvalidDocument) from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/bson-1.6.1/lib/bson/bson_c.rb:24:in `serialize' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongo-1.6.1/lib/mongo/cursor.rb:604:in `construct_query_message' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongo-1.6.1/lib/mongo/cursor.rb:465:in `send_initial_query' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongo-1.6.1/lib/mongo/cursor.rb:458:in `refresh' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongo-1.6.1/lib/mongo/cursor.rb:128:in `next' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongo-1.6.1/lib/mongo/db.rb:509:in `command' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongo-1.6.1/lib/mongo/cursor.rb:191:in `count' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongoid-2.4.6/lib/mongoid/cursor.rb:42:in `block in count' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongoid-2.4.6/lib/mongoid/collections/retry.rb:29:in `retry_on_connection_failure' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongoid-2.4.6/lib/mongoid/cursor.rb:41:in `count' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongoid-2.4.6/lib/mongoid/contexts/mongo.rb:93:in `count' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongoid-2.4.6/lib/mongoid/criteria.rb:45:in `count' from /home/scepion1d/Workspace/RubyMine/dana-x/.bundle/ruby/1.9.1/gems/mongoid-2.4.6/lib/mongoid/finders.rb:60:in `exists?' from /home/scepion1d/Workspace/RubyMine/dana-x/crawler/crawler.rb:110:in `block (2 levels) in push_entries_to_db' from /home/scepion1d/Workspace/RubyMine/dana-x/crawler/crawler.rb:103:in `each' from /home/scepion1d/Workspace/RubyMine/dana-x/crawler/crawler.rb:103:in `block in push_entries_to_db' from /home/scepion1d/Workspace/RubyMine/dana-x/crawler/crawler.rb:102:in `each' from /home/scepion1d/Workspace/RubyMine/dana-x/crawler/crawler.rb:102:in `push_entries_to_db' from main_starter.rb:15:in `<main>' Can anyone tell what am I doing wrong?