如果使用$cond通过$project管道支持操作员的当前Spring Data版本，则可以将其转换为（未测试）：
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$cond在聚合操作中不支持运算符的Spring-Data MongoDB版本，有一种解决方法是实现AggregationOperation接口以接收DBObject：
public class CustomProjectAggregationOperation implements AggregationOperation {
private DBObject operation;
public CustomProjectAggregationOperation (DBObject operation) {
this.operation = operation;
}
@Override
public DBObject toDBObject(AggregationOperationContext context) {
return context.getMappedObject(operation);
}
}
然后$project，在聚合管道中将操作实现为DBObject，该操作与你拥有的操作相同：
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);
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