I'm trying to sort values after executing a find on my MongoDB by Java API. The result list contains the following entries:
{
"_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:
DBCursor cursor = getCollection().find(query);
BasicDBObject orderBy = new BasicDBObject("labels[0].value", 1);
cursor.sort(orderBy);
Cursor values are not sorted by this code. Can you help me?
Have you tried
BasicDBObject orderBy = new BasicDBObject("labels.0.value", 1);
It's not obvious, but the MongoDB documentation eludes to it. Using the $ sign matches the first item, but specifying the array element number seems to work. If anyone has a better document describing the behavior, please reply with the link.
From the documentation
Update Documents in an Array
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.$.field" : value } } )
Documentation is here
I found this question/answer when looking for How to sort MongoDB (in Meteor) by first item of array…
I used this to find all documents in the Schedules
collection which are active
and then sort ascending by the first day in the days
array.
Schedules.find(
{
active: true,
},
{
sort: { 'days.0': 1 },
},
)
A sample Schedule
collection document with a Monday, Wednesday, Friday schedule looks something like this:
{
_id: 9dh3ld0d7d0d,
userId: UJD9dKd8edo,
active: true,
days: [1, 3, 5],
}
You cannot actually "sort" by a specific index of an array within a document in MongoDB. ct If you really must do this then you need the aggregation framework to "extract" the element to sort on.
I know the list form is actually deprecated, so this code is just for demonstration. Acutally define your pipeline as individual variables and feed those as argument to aggregate:
BasicDBList pipeline = new BasicDBList();
list.add(new BasicDBObject("$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:
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:
collection.aggregate(unwind,group,sort);
Where those are individually declared.
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