[英]Run an aggregation using Java Driver - MongoDB
使用MongoDB shell,我使用:
db.bios.aggregate(
[
{$match:{"contribs.0.name":{"$exists":1}}},
{$project: {contribs:{$arrayElemAt:["$contribs",0]}}}
]
)
如何使用Java驱动程序( 2.14.1 )进行相同的查询? 我尝试:
首先,我为$match
阶段创建一个DBObject:
DBObject match = new BasicDBObject("$match",new BasicDBObject("contribs.0.name",
new BasicDBObject("$exists",1)));
然后我创建一个BasicDBList
:
BasicDBObject obj = new BasicDBObject("$contribs",0);
BasicDBList arrayElemAt = new BasicDBList();
arrayElemAt.add(obj);
这是$project
阶段:
DBObject project1 = new BasicDBObject("$project", new BasicDBObject("contribs",
new BasicDBObject("$arrayElemAt",arrayElemAt)));
最后,我创建了聚合管道:
List<DBObject> list = new ArrayList<>();
list.add(match);
list.add(project1);
AggregationOutput output = this.coll.aggregate(list);
$Match
阶段有效,但$project
无效。
我收到一个错误: "errmsg" : "invalid operator '$contribs'" , "code" : 15999
您本打算创建List
时创建了一个DBObject
。 此外, DBList
已经过时了一段时间。 习惯使用标准列表符号:
List<DBObject> pipeline = Arrays.<DBObject>asList(
new BasicDBObject(
"$match",
new BasicDBObject(
"contribs.0", new BasicDBObject("$exists",true)
)
),
new BasicDBObject(
"$project",
new BasicDBObject(
"contribs", new BasicDBObject("$arrayElemAt", Arrays.asList("$contribs",0))
)
)
);
AggregationOutput output = this.coll.aggrgate(pipeline);
还要注意,在现代驱动程序中,您实际上应该使用Document
代替所有DBObject
类型。
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