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

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 Hotel class anymore. Because $unwind operation 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 Hotel like 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);

Answer 2

I had this problem and i solve it by using the "project" pipeline after unwind. in spring boot aggregation this will happen but in the mongo shell, it won't need a project after unwind

Answer 3

It occurs to me a very different problem making me crazy for long time a day ago and I think it could be very useful to share here even if it is not about unwind:

• at the beginning I design a model with java + mongoDB in which I put some fields in a key inside a document and made several tests, for example {"header":{"key":"first", "value": 4}}
• then I changed my mind and think not to have simple nested objects but a list of them to be more general such as {"header":[{"key":"first", "value": 4}] }

When I was working only with new documents all was running fine, but then making more general tests I continuously had crashes with this mongodb error about List because in my DB I had old documents without list but with simple objects I forgot to remove and which were in conflict with the new model wanting a list.

Simple had I to clean DB and starting tests from scratch.