def to_mongo
      self.to_hash.inject({}) do |hash, (place_in_statement, entity)| 
        hash.merge(RDF::Mongo::Conversion.to_mongo(entity, place_in_statement)) 
      end
    end
def to_mongo
      self.to_hash.inject({}) do |hash, (place_in_statement, entity)| 
        hash.merge(RDF::Mongo::Conversion.to_mongo(entity, place_in_statement)) 
      end
    end
Creates a BSON representation of the statement.
 @return [Hash]