Spring-data-mongo unable to instantiate java.util.List using Constructor

Question

Using spring-data-mongodb-1.5.4 and mongodb-driver-3.4.2

I've a class Hotel

    public class Hotel {

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

Review class :

public class Review {

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

When I am calling Aggregation.unwind("reviews"); it throws

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);

I see this question but does't help me.

How to resolve this?

Answer 1

When you $unwind reviews field, query's return json structure does not match with your Hotelclass anymore. Because $unwindoperation makes reviews a sub object instead of a list. If you try your query in robomongo or some other tools, you can see your return object is like that

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

So you should use another class instead of Hotellike 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);