Wrong reference in Spring MongoDB data aggregation - mongodb

Question

I have mongodb documents of following kind:
{
"_id" : {
"refId" : ObjectId("55e44dd70a975a6fec7ae66e"),
"someString" : "foo"
},
"someValue" : 1,
"date" : NumberLong("1441025536869"),
"subdoc" : {
"someRef" : ObjectId("xf2h55e44dd70a975a6fec7a"),
"count" : 99
}
}
Want to get list of documents with unique "subdoc.someRef" with earliest "date" (if there are any documents with same "subdoc.someRef") and sorted by any field. The issue is in sorting by field "subdoc.count".
I use following spring data aggregation:
Aggregation agg = Aggregation.newAggregation(
Aggregation.match(match),
Aggregation.project("date", "someValue")
.and("subdoc.someRef").as("someRef")
.and("subdoc.count").as("count")
.and("_id.someString").as("someString")
.and("_id.refId").as("refId"),
Aggregation.sort(Sort.Direction.ASC, "date"),
Aggregation.group("someRef")
.min("date").as("date")
.first("refId").as("refId")
.first("someValue").as("someValue")
.first("count").as("count"),
Aggregation.project("someValue", "date", "refId", "count")
.and("_id").as("someRef"),
Aggregation.sort(pageable.getSort()),
Aggregation.skip(pageable.getOffset()),
Aggregation.limit(pageable.getPageSize())
);
Everething is fine except how spring data converts: .first("count").as("count")
I always got "count" : null in aggregation result.
DBObject created by spring is not what I've expected. That line in log concerns me:
"$group" : { "_id" : "$someRef" , "date" : { "$min" : "$date"} , "refId" : { "$first" : "$_id.refId"} , "someValue" : { "$first" : "$someValue"} , "count" : { "$first" : "$subdoc.count"}}}
I cannot understand why it always puts "$subdoc.count" instead of putting "$count" as result of previous step of aggreagation pipeline. So "count" in result is always null as "$subdoc.count" is always null in last grop step.
How to make Spring use the value that I want instead of putting reference?

Answer 1

Thanks to comment, the solution is to eliminate projections and updating group stage. Grouping work well with subdocument references, that I didn't expected:
Aggregation agg = Aggregation.newAggregation(
Aggregation.match(match),
Aggregation.sort(Sort.Direction.ASC, "date"),
Aggregation.group("subdoc.someRef")
.first("subdoc.count").as("count")
.first("_id.refId").as("refId")
.first("_id.someString").as("someString")
.first("date").as("date")
.first("someValue").as("someValue"),
Aggregation.sort(pageable.getSort()),
Aggregation.skip(pageable.getOffset()),
Aggregation.limit(pageable.getPageSize())
);