old_contents: from flask_restful import fields
  
  from .feedback import FeedbackResource
  
  CoffeeResource = {
      'id': fields.Integer,
      'vendor_id': fields.Integer,
      'name': fields.String,
      'on_sale_date': fields.DateTime,
      'imported': fields.Boolean,
      'feedbacks': fields.List(fields.Nested(FeedbackResource))
  }
  
new_contents: from flask_restful import fields
  
  from .feedback import FeedbackResource
  
  CoffeeResource = {
      'id': fields.Integer,
      'vendor_id': fields.Integer,
      'name': fields.String,
      'on_sale_date': fields.DateTime(dt_format="iso8601"),
      'imported': fields.Boolean,
      'feedbacks': fields.List(fields.Nested(FeedbackResource))
  }
  
subject: add datetime format