code: function toLink( data ) {
  	if ( data && typeof data === 'object' ) {
  		if ( data.constructor.name === 'ObjectID' || data.constructor.name === 'ObjectId' ) {
  			return { _id: data } ;
  		}
  
  		data._id = toObjectId( data._id ) ;
  	}
  	else if ( typeof data === 'string' ) {
  		try {
  			data = { _id: mongodb.ObjectID( data ) } ;
  		}
  		catch ( error ) {}
  	}
  
  	return data ;
  }
docs: Allow one to pass the whole linked target object