java - Spring-data-mongo 无法使用构造函数实例化 java.util.List

Question

使用 spring-data-mongodb-1.5.4 和 mongodb-driver-3.4.2

我有一个类旅馆

    public class Hotel {

        private String name;
        private int pricePerNight;
        private Address address;
        private List<Review> reviews;
//getter, setter, default constructor, parameterized constructor 

复习类:

public class Review {

    private int rating;
    private String description;
    private User user;
    private boolean isApproved;
 //getter, setter, default constructor, parameterized constructor 

当我调用 Aggregation.unwind("reviews"); 它抛出

org.springframework.data.mapping.model.MappingInstantiationException: Failed to instantiate java.util.List using constructor NO_CONSTRUCTOR with arguments

UnwindOperation unwindOperation = Aggregation.unwind("reviews");
Aggregation aggregation = Aggregation.newAggregation(unwindOperation);
AggregationResults<Hotel> results=mongoOperations.aggregate(aggregation,"hotel", Hotel.class);

我看到 this question但对我没有帮助。

如何解决？

Answer 1

当您在 $unwind reviews 字段时，查询的返回 json 结构不再与您的 Hotel 类匹配。因为 $unwindoperation 使 reviews 成为子对象而不是列表。如果你在 robomongo 或其他一些工具中尝试你的查询，你可以看到你的返回对象是这样的

{
  "_id" : ObjectId("59b519d72f9e340bcc830cb3"),
  "id" : "59b23c39c70ff63135f76b14",
  "name" : "Signature",
  "reviews" : {
    "id" : 1,
    "userName" : "Salman",
    "rating" : 8,
    "approved" : true
  }
}

所以你应该使用另一个类而不是 Hotel，比如 UnwindedHotel

public class UnwindedHotel {

    private String name;
    private int pricePerNight;
    private Address address;
    private Review reviews;
}

UnwindOperation unwindOperation = Aggregation.unwind("reviews");
Aggregation aggregation = Aggregation.newAggregation(unwindOperation);
AggregationResults<UnwindedHotel> results=mongoOperations.aggregate(aggregation,"hotel", UnwindedHotel.class);