[英]MongoDB, Java, sort by first array entry
I'm trying to sort values after executing a find on my MongoDB by Java API. 我正在尝试通过Java API在MongoDB上执行查找后对值进行排序。 The result list contains the following entries: 结果列表包含以下条目:
{
"_id": "P17-223",
"property": "P17",
"itemid": 223,
"labels": [
{
"language": "en",
"value": "Greenland"
},
{
"language": "es",
"value": "Groenlandia"
},
{
"language": "de",
"value": "Grönland"
}
]
} }
I want to sort by the first entry of the array labels : 我想按数组标签的第一个条目排序:
DBCursor cursor = getCollection().find(query);
BasicDBObject orderBy = new BasicDBObject("labels[0].value", 1);
cursor.sort(orderBy);
Cursor values are not sorted by this code. 此代码不对游标值进行排序。 Can you help me? 你能帮助我吗?
Have you tried 你有没有尝试过
BasicDBObject orderBy = new BasicDBObject("labels.0.value", 1);
It's not obvious, but the MongoDB documentation eludes to it. 这并不明显,但是MongoDB文档却没有提到它。 Using the $ sign matches the first item, but specifying the array element number seems to work. 使用$符号匹配第一个项目,但指定数组元素编号似乎有效。 If anyone has a better document describing the behavior, please reply with the link. 如果有人有更好的文档描述行为,请回复链接。
From the documentation 从文档中
Update Documents in an Array 更新阵列中的文档
The positional $ operator facilitates updates to arrays that contain embedded
documents. Use the positional $ operator to access the fields in the embedded
documents with the dot notation on the $ operator.
db.collection.update( { <query selector> }, { <update operator>: { "array.$.field" : value } } )
You cannot actually "sort" by a specific index of an array within a document in MongoDB. 实际上,您无法通过MongoDB中文档中特定的数组索引进行“排序”。 ct If you really must do this then you need the aggregation framework to "extract" the element to sort on. ct如果你真的必须这样做,那么你需要聚合框架来“提取”要排序的元素。
I know the list form is actually deprecated, so this code is just for demonstration. 我知道列表表单实际上已弃用,因此此代码仅用于演示。 Acutally define your pipeline as individual variables and feed those as argument to aggregate: 将您的管道实际定义为单个变量,并将它们作为参数提供给聚合:
BasicDBList pipeline = new BasicDBList();
list.add(new BasicDBObject("$unwind","$labels"));
list.add(new BasicDBObject("$group",
new BasicDBObject("_id","$_id")
.append("property", new BasicDBObject("$first","$property"))
.append("itemid", new BasicDBObject("$first","$itemid"))
.append("labels", new BasicDBObject("$push","$labels"))
.append("maxLabel", new BasicDBObject("$max", "$labels.value"))
));
list.add(new BasicDBObject("$sort", new BasicDBObject("maxLabel",1)));
System.out.println(pipeline);
That gives you the serialized version which is the JSON form of: 这为您提供了序列化版本,它是JSON形式:
db.collection.aggregate([
{ "$unwind" : "$labels" },
{ "$group": {
"_id": "$_id",
"property": { "$first" : "$property" },
"itemid": { "$first" : "$itemid" },
"labels": { "$push" : "$labels" },
"maxLabel": { "$max" : "$labels.value"}
}},
{ "$sort" : { "maxLabel" : 1} }
])
Better applied in your code as: 更好地应用于您的代码:
collection.aggregate(unwind,group,sort);
Where those are individually declared. 那些是单独宣布的。
I found this question/answer when looking for How to sort MongoDB (in Meteor) by first item of array … 我在寻找如何通过第一项数组对MongoDB(在Meteor中)进行排序时找到了这个问题/答案...
I used this to find all documents in the Schedules
collection which are active
and then sort ascending by the first day in the days
array. 我用这个来查找所有文件Schedules
收集了一些active
,然后排序的第一天升days
阵。
Schedules.find(
{
active: true,
},
{
sort: { 'days.0': 1 },
},
)
A sample Schedule
collection document with a Monday, Wednesday, Friday schedule looks something like this: 带有星期一,星期三,星期五计划的示例Schedule
集合文档如下所示:
{
_id: 9dh3ld0d7d0d,
userId: UJD9dKd8edo,
active: true,
days: [1, 3, 5],
}
Since MongoDB 3.0 Java Api, you can use com.mongodb.client.model.Sorts 从MongoDB 3.0 Java Api开始,您可以使用com.mongodb.client.model.Sorts
Exemple use : 例如:
col.find().sort(Sorts.orderBy(Sorts.descending(fieldname)));
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