如何使用 mongodb java-driver Projections.slice

Question

I am trying to use Aggregates.project to slice the array in my documents. My documents is like

{
"date":"",
"stype_0":[1,2,3,4]
}

in the mongochef looks like

and my code in java is :

Aggregates.project(Projections.fields(
                                Projections.slice("stype_0", pst-1, pen-pst),Projections.slice("stype_1", pst-1, pen-pst),
                                Projections.slice("stype_2", pst-1, pen-pst),Projections.slice("stype_3", pst-1, pen-pst))))

finally i get error

First argument to $slice must be an array, but is of type: int

I guess that is because the first element in stype_0 is int , but I really do not know why? Thanks a lot!

Answer 1

Slice has two versions. $slice(aggregation) & $slice(projection) . You are using the wrong one.

Aggregate slice function doesn't have any built-in support. Below is an example for one such projection. Do the same for all the other projection fields.

List stype_0 = Arrays.asList("$stype_0", 1, 1);    
Bson project = Aggregates.project(Projections.fields(new Document("stype_0", new Document("$slice", stype_0))));
AggregateIterable<Document> iterable = dbCollection.aggregate(Arrays.asList(project));