Spring Data MongoDb-查找所有嵌套對象

Question

假設我有以下兩個文檔：

 @Document
 public class Exam {
   private int examId;
   private List<Question> questions;

和

 public class Question {
   private int questionId;
   private String question;

我需要編寫一個“ findAll”，使用MongoRepository或Java中的某些其他方式，為某個“ Exam”對象（examId == n）返回所有問題的列表（或者理想情況下，在Question對象中僅返回“ question”字符串） Spring Data MongoDb，我該怎么做？

{
    "_id" : ObjectId("xyz"),
    "_class" : "com.xxx.Exam",
    "examId" : 1,
    "questions" : [ 
        {"questionId" : 1, "question" : "xyz" },
        {"questionId" : 2, "question" : "abc" }
    ]
}

Answer 1

有多種方法可以實現，其中一種可能是這樣的：

MatchOperation match = match(new Criteria("examId").is(1));
UnwindOperation unwind = unwind("questions");
ProjectionOperation project = project().andExclude("_id").and("questions.question").as("question");
Aggregation aggregation = newAggregation(match, unwind, project);
AggregationResults<DBObject> result = mongoOperations.aggregate(aggregation, "exams", DBObject.class);

result.forEach(new Consumer<DBObject>() {
    @Override
    public void accept(DBObject t) {
        System.out.println(t.toString());
    }
});
// otuput
// { "question" : "xyz"}
// { "question" : "abc"}

這里將結果映射到DBObject，但是您可以定義更合適的類。