spring - Spring-MongoDb聚合框架中如何使用$cond操作

Question

我有一个聚合管道，其中包括这样的项目:

$project: {
  start: {
    $cond: {
      if: {
        $eq: ["$start", "EARLY"]
      },
      then: "$deltastart.start",
      else: "$deltastart.end"
    }
  },...
},...

在 mongo shell 中运行良好。如何使用 Spring-Mongodb 中的聚合框架来表达这一点？我见过 ProjectionOperationBuilder、ExpressionProjectionOperationBuilder 类型，但没有看到如何使用它们的示例……有什么建议吗？

Answer 1

如果使用支持 $cond 的当前 Spring Data 版本 运算符通过 $project 管道，然后可以将其转换为(未经测试):

import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
import static org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond.*;
import org.springframework.data.mongodb.core.query.Criteria;

Cond condOperation = ConditionalOperators.when(Criteria.where("start").is("EARLY"))
                                    .thenValueOf("deltastart.start")
                                    .otherwise("deltastart.end");

Aggregation agg = newAggregation(project().and(condOperation).as("start"));
AggregationResults<MyClass> results = mongoTemplate.aggregate(agg, MyClass.class); 
List<MyClass> myList = results.getMappedResults();

---

对于不支持 $cond 的 Spring-Data MongoDB 版本 运算符在聚合操作中，有一个变通方法是实现 AggregationOperation 接收 DBObject 的接口(interface):

public class CustomProjectAggregationOperation implements AggregationOperation {
    private DBObject operation;

    public CustomProjectAggregationOperation (DBObject operation) {
        this.operation = operation;
    }

    @Override
    public DBObject toDBObject(AggregationOperationContext context) {
        return context.getMappedObject(operation);
    }
}

然后将 $project 操作实现为聚合管道中的 DBObject，与您拥有的相同:

DBObject operation = (DBObject) new BasicDBObject(
    "$project", new BasicDBObject(
         "start", new BasicDBObject(
                "$cond", new Object[]{
                        new BasicDBObject(
                            "$eq", new Object[]{ "$start", "EARLY"}
                        ),
                        "$deltastart.start",
                        "$deltastart.end"
                 }
           )
     )
);

然后您可以在 TypeAggregation 中使用它:

TypedAggregation<CustomClass> aggregation = newAggregation(CustomClass.class,
    new CustomProjectAggregationOperation(operation)
);
AggregationResults<CustomClass> result = mongoTemplate.aggregate(aggregation, CustomClass.class);