MongoDB shell aggregation does not work in Morphia - mongodb

Question

Consider the following MongoDB collection:
{
"_id" : ObjectId("..."),
"myId": 12345,
"root": {
basicData: {
code: "CODE"
}
data: [
{
descriptions: {
description: [
{
text: "...",
language: "de"
}
]
}
}
]
}}
I'm trying to get documents filtered by "myId" and "code", but with descriptions in only one specific language. In the shell, the following command seems to work properly:
db.Items.aggregate([
{ "$match" : { "myId" : 40943 , "root.basicData.code" : "A_CODE"}},
{ "$unwind" : "$root.data"},
{ "$unwind" : "$root.data.descriptions.description"},
{ "$match" : { "root.data.descriptions.description.language" : "de"}}
])
In Morphia I try to do the following to get to the same result:
AggregationPipeline pipeline = dataStore.createAggregation(Item.class);
Query<Item> matchIdAndCode = dataStore.createQuery(Item.class);
matchIdAndCode.field("myId").equal(myid);
matchIdAndCode.field("root.basicData.code").equal(code);
pipeline.match(matchIdAndCode);
pipeline.unwind("root.data");
pipeline.unwind("root.data.descriptions.description");
Query<Item> matchLanguage = dataStore.createQuery(Item.class);
matchLanguage.field("root.data.descriptions.description.language").equal(language);
pipeline.match(matchLanguage);
Iterator<Item> itemAggregate = pipeline.aggregate(Item.class);
but the iterator does not contain any items. I am not shure where to search for further errors, especially because when I copy the stages in the morphia aggregation pipeline to the shell, I get the expected result.

Answer 1

you are missing $ sign in following lines
pipeline.unwind("root.data");
pipeline.unwind("root.data.descriptions.description");
should be
pipeline.unwind("$root.data");
pipeline.unwind("$root.data.descriptions.description");

Answer 2

As a workaround, I now used the MongoDB Java Driver. My working solution:
List<DBObject> stages = new ArrayList<DBObject>();
DBCollection collection = dataStore.getCollection(Item.class);
// match
DBObject matchFields = new BasicDBObject("myId", myid);
matchFields.put("code", code);
DBObject match = new BasicDBObject("$match", matchFields );
stages.add(match);
// unwind
DBObject unwindDescriptiveData = new BasicDBObject("$unwind", "$root.data");
stages.add(unwindDescriptiveData);
DBObject unwindDescription = new BasicDBObject("$unwind", "$root.data.descriptions.description");
stages.add(unwindDescription);
// match
DBObject languageMatchFields = new BasicDBObject("root.data.descriptions.description.language", language);
DBObject languageMatch = new BasicDBObject("$match", languageMatchFields );
stages.add(languageMatch);
AggregationOutput aggregate = collection.aggregate(stages);
The mapping to the pojo can be done with Morphia again:
List<Item> items = new ArrayList<Item>();
for (Iterator<DBObject> iterator = aggregate.results().iterator(); iterator.hasNext();) {
items.add(morphia.fromDBObject(Item.class, iterator.next()));
}