Java MongoDB Projection - mongodb

Question

I am referring mongodb official page for projection where I came across following example where elements of array in subdocument is filtered:
https://docs.mongodb.com/manual/reference/operator/aggregation/filter/#exp._S_filter
db.sales.aggregate([
{
$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: { $gte: [ "$$item.price", 100 ] }
}
}
}
}
])
I am trying to implement this in Java but I am not doing it correctly and elements in subdocument array are not filtered.
Input Collection:
{
_id: 0,
items: [
{ item_id: 43, quantity: 2, price: 10 },
{ item_id: 2, quantity: 1, price: 240 }
]
}
{
_id: 1,
items: [
{ item_id: 23, quantity: 3, price: 110 },
{ item_id: 103, quantity: 4, price: 5 },
{ item_id: 38, quantity: 1, price: 300 }
]
}
{
_id: 2,
items: [
{ item_id: 4, quantity: 1, price: 23 }
]
}
Expected Output Collection:
{
"_id" : 0,
"items" : [
{ "item_id" : 2, "quantity" : 1, "price" : 240 }
]
}
{
"_id" : 1,
"items" : [
{ "item_id" : 23, "quantity" : 3, "price" : 110 },
{ "item_id" : 38, "quantity" : 1, "price" : 300 }
]
}
{ "_id" : 2, "items" : [ ] }
In Java(mongo Driver 3.9.1), this is what I am doing:
Bson priceFilter = Filters.gte("items.price", 100);
mongoCollection.aggregate(
Aggregates.project(Projections.fields(priceFilter))
);
How do I project with aggregate function for the subdocument arrays where I need to filter out elements from subdocument array based on some condition?

Answer 1

In MongoDB Java Driver 3.9.1, collection.aggregate() takes a java.util.List as parameter. So you need to replace your Java code with the below.
mongoCollection.aggregate(
Arrays.asList(
Aggregates.project(Projections.computed("items",
new Document().append("$filter",
new Document().append("input", "$items").append("as", "item").append("cond",
new Document().append("$gte", Arrays.asList("$$item.price",100))))))
)
);