How to get Spring Data Mongo Aggregations to work like they do with mongo - mongodb

Question

I am attempting a simple projection using Spring Data Mongo's Aggregation API.
The pipeline step I want to do is:
{
$project : {
"account._id" : 1,
"account.position" : 1
}
}
This is what I have tried (along with a ton of other tweaks because nothing seems to work):
ProjectionOperation project1 = Aggregation.project("account._id", "account.position");
However, even though this is how the documentation says to do it here: https://docs.spring.io/spring-data/mongodb/docs/current/reference/html/#mongo.aggregation.projection
The actual document that is rendered by that projection ends up looking like:
{
$project : {
_id : "$account._id",
position : "$account.position"
}
}
Which works completely differently than the projection that I am wanting to use.
Does anyone know how to get a projection like I want out of Spring Data Mongo Aggregation API, or is this a bug I need to report?
Update 8/29/2019 - Adding more data to build out the context:
Two collections are involved: "groups" and "accounts"
A group looks something like this:
{
_id : ObjectId("..."),
name: ...,
ownerId: ObjectId("..."),
other stuff...
}
An account looks something like this:
{
_id : ObjectId("..."),
position : "ABC",
memberships : [{
groupId: ObjectId("..."),
otherstuff: ...,
}],
other stuff...
}
My whole aggregation looks like this and works as desired in mongodb shell: (trying to get a list of all account ids of a particular type that are members of any groups owned by a particular user)
groups.aggregate(
{
$match : {
ownerId : ObjectId("XYZ"),
}
},
{
$lookup: {
from: "accounts",
localField: "_id",
foreignField: "memberships.groupId",
as: "account"
}
},
{
$project: {
"account._id" : 1,
"account.position" : 1
}
},
{
$unwind: "$account"
},
{
$match: {
"account.position" : "ZZZ"
}
},
{
$project: {
_id : 0,
accountId : "$account._id"
}
})
Java version of the Aggregation:
MatchOperation match1 = Aggregation.match(
where("ownerId").is(accountId));
LookupOperation lookupOperation = LookupOperation.newLookup()
.from("accounts")
.localField("_id")
.foreignField("memberships.groupId")
.as("account");
// This doesn't work correctly on nested fields:
ProjectionOperation project1 = Aggregation.project(
"studentAccount._id",
"studentAccount.position");
Aggregation aggregation = Aggregation.newAggregation(
match1,
lookupOperation,
project1,
unwind("account"),
match(where("account.position").is("ZZZ")),
project().and("account._id").as("accountId"));

Answer 1

If you want your aggregation work look like mongoshell your could try like this
Aggregation aggregation = Aggregation.newAggregation(
match1,
lookupOperation,
// This's your project operation
new AggregationOperation() {
#Override
public Document toDocument(AggregationOperationContext aggregationOperationContext) {
Document project = new Document("$project",
new Document(
"_id", "$account._id"
).append("position", "$account.position")
);
return aggregationOperationContext.getMappedObject(project);
}
},
unwind("account"),
match(where("account.position").is("ZZZ")),
project().and("account._id").as("accountId")
);
You can check my answer here in a more generic way