mongodbtemplate 关联表查询报错

Question

我有两个表：url和userClickUrl表，字段如下
url:
String id;
String uid;
String title;

userClickUrl:
String id;
// 跟url中的id关联
String urlId;
String uid;
int clickCount;

现在我想使用userClickUrl为主表，url为子表，根据clickCount排序，查询url数据，但是报错，下面是java代码：

LookupOperation lookupOperation = Aggregation.lookup("url", "urlId", "_id", "url_as");
AggregationOperation match = Aggregation.match(Criteria.where("uid").is(uid));
ProjectionOperation project = Aggregation.project(UserClickUrl.class);
SortOperation sort = Aggregation.sort(Sort.Direction.DESC, "clickCount");
LimitOperation limit = Aggregation.limit(10);
operationList.add(lookupOperation);
operationList.add(match);
operationList.add(addFieldsOperation);
operationList.add(project);
operationList.add(sort);
operationList.add(limit);
Aggregation agg = Aggregation.newAggregation(operationList);
AggregationResults<UserClickUrl> aggregationResults = mongoTemplate.aggregate(agg, "userClickUrl", UserClickUrl.class);
List<UserClickUrl> dataList = aggregationResults.getMappedResults();

2021-09-22 09:50:26.182 ERROR 2119 --- [nio-8902-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.IllegalArgumentException: Invalid reference 'clickCount'!] with root cause

java.lang.IllegalArgumentException: Invalid reference 'clickCount'!
        at org.springframework.data.mongodb.core.aggregation.ExposedFieldsAggregationOperationContext.getReference(ExposedFieldsAggregationOperationContext.java:114) ~[spring-data-mongodb-3.1.3.jar:3.1.3]
        at org.springframework.data.mongodb.core.aggregation.ExposedFieldsAggregationOperationContext.getReference(ExposedFieldsAggregationOperationContext.java:86) ~[spring-data-mongodb-3.1.3.jar:3.1.3]

Answer 1

我把上面的代码，换成下面的就好了

ProjectionOperation project = Aggregation.project(UserClickUrl.class);

换成：

ProjectionOperation project = Aggregation.project("uid", "urlId", "clickCount");

是不是就是说Aggregation.project不能使用.class表示，只能通过字符串表示字段field