Morphia java.util.Arrays $ ArrayList在进行Projection.projection时无法转换为com.mongodb.DBObject。

Question

我正在尝试进行以下汇总

db.getCollection("order").aggregate(
    [
        { 
            "$project" : {
                "_id" : -1.0, 
                "customerId" : "$customer.customerId", 
                "hasOrderInT0" : {
                    "$cond" : [
                        {
                            "$and" : [
                                {
                                    "$gte" : [
                                        "$date", 
                                        1577829600.0
                                    ]
                                }, 
                                {
                                    "$lte" : [
                                        "$date", 
                                        1580507999.0
                                    ]
                                }
                            ]
                        }, 
                        1, 
                        0
                    ]
                }
            }
        }
    ]
);

在Java应用中，我使用Morphia作为ORM。基本上，如果日期在2个时间戳之间，它将在hasOrderInT0字段中输入1，否则将0。 

long initialStart = 1577829600;
long initialEnd = 1580507999;

AggregationPipeline pipeline = databaseService.getConnection().createAggregation(Order.class)
                .project(
                        Projection.projection("_id", "-1"),
                        Projection.projection("customerId", "$customer.customerId"),
                        Projection.projection("hasOrderInT0",
                                Projection.expression(
                                        "$cond",
                                        Arrays.<Object>asList(
                                                new BasicDBObject(
                                                        "$and", Arrays.<Object>asList(
                                                        new BasicDBObject(
                                                                "$gte", Arrays.<Object>asList("$date", initialStart)
                                                        ),
                                                        new BasicDBObject(
                                                                "$lte", Arrays.<Object>asList("$date", initialEnd)
                                                        )
                                                )
                                                ),
                                                1,
                                                0
                                        )
                                )
                        )
                );

运行上面的代码时，出现以下错误：

Caused by: java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to com.mongodb.DBObject
    at xyz.morphia.aggregation.AggregationPipelineImpl.toExpressionArgs(AggregationPipelineImpl.java:296)
    at xyz.morphia.aggregation.AggregationPipelineImpl.toDBObject(AggregationPipelineImpl.java:249)
    at xyz.morphia.aggregation.AggregationPipelineImpl.toDBObject(AggregationPipelineImpl.java:240)
    at xyz.morphia.aggregation.AggregationPipelineImpl.project(AggregationPipelineImpl.java:191)

这是我第一次将Projection与Morphia结合使用，我不知道这是否是实现可在mongo控制台中使用的命令的正确方法。

PS：$project只是来自较大聚合的管道，但这是您感兴趣的部分，并且正在给出错误，因此出于演示目的，我对其进行了简化。

我正在尝试放置以下聚合db.getCollection（“ order”）。aggregate（[[“” $ project“：{” _id“：-1.0，” customerId“：” $ customer ....

Answer 1

原来没有必要将$ cond的条件包装到Arrays.<Object>asList中。 Projection.expression已经接受任意数量的参数，因此工作代码为：

long initialStart = 1577829600;
long initialEnd = 1580507999;

AggregationPipeline pipeline = databaseService.getConnection().createAggregation(Order.class)
                .project(
                        Projection.projection("_id", "-1"),
                        Projection.projection("customerId", "$customer.customerId"),
                        Projection.projection("hasOrderInT0",
                                Projection.expression(
                                        "$cond",
                                            new BasicDBObject(
                                                    "$and", Arrays.<Object>asList(
                                                    new BasicDBObject(
                                                            "$gte", Arrays.<Object>asList("$date", initialStart)
                                                    ),
                                                    new BasicDBObject(
                                                            "$lte", Arrays.<Object>asList("$date", initialEnd)
                                                    )
                                            )
                                            ),
                                            1,
                                            0
                                )
                        )
                );