Spring mongodb: Aggregation with lookup, projection and slice

Question

I have a collection ref consisting of

{
  "_id": "refId"
  "ref": "itemId"
}

and a collection item

{
  "_id": "itemId"
  "field1": ...
  "array": [
    {
      "field2": "a"
    }, 
    {
      "field2": "b"
    }
  ]
}

Here i need to 'join' a.k.a perform a lookup on ref to item, and project the fields field1 and the last item of array as arrayItem into the top level of the return document, to return

{
  "_id": "itemId",
  "field1": ...
  "arrayItem: "b"
}

Using the mongo shell, this works perfectly using the following statement:

db.ref.aggregate([
  { "$lookup": {
      "from": "item",
      "localField": "ref",
      "foreignField": "_id",
      "as": "item"
    }},
  { "$unwind": "$item" },
  { "$project": {
      "_id": "$item._id",
      "field1": "$item.field1",
      "arrayItem": { $slice: [ "$item.array", -1, 1 ]}
    }},
  { "$unwind": "$arrayItem" }
])

Now, i have to make this happen in Java with Spring and Spring-MongoDB, which i have tried using this Aggregation:

Aggregation.newAggregation(
    new LookupAggreationOperation("item", "ref", "_id", "item"),
    Aggregation.unwind("item"),
    Aggregation.project(Fields.from(
        Fields.field("_id", "$item._id"),
        Fields.field("field1", "$item.field1"),
        Fields.field("array", "$item.array"))),
    Aggregation.project("_id", "field1", "array")
        .and("$array").project("slice", -1, 1).as("$arrayItem"),
    Aggregation.unwind("$array"));

As lookup is only available in Spring-Mongo 1.9.2, I had to rebuild it myself like that:

public class LookupAggregationOperation implements AggregationOperation {
    private DBObject operation;

    public LookupAggregationOperation(String from, String localField, 
                                      String foreignField, String as) {
        this.operation = new BasicDBObject("$lookup", //
                new BasicDBObject("from", from) //
                        .append("localField", localField) //
                        .append("foreignField", foreignField) //
                        .append("as", as));
    }

    @Override
    public DBObject toDBObject(AggregationOperationContext context) {
        return context.getMappedObject(operation);
    }
}

The problem is that the slice command is not yet implemented as method in Aggregation (and isn't in 1.9.2), hence i need to call project("slice", -1, 1).as("$arrayItem") as suggested in DATAMONGO-1457. The slicing is simply not executed, arrayItem is null in the result.

I'm using Spring 1.3.5 with Spring MongoDB 1.8.4 and MongoDB 3.2.8.