MongoDB Spring Data gives No property '$$value' found on class exception - mongodb

Question

//Sample Collection
{
//fields
"roleList" : [ROLE_1, ROLE_2, ROLE_3],
"siteList" : [
{
"role" : ROLE_1,
//fields
},
{
"role" : ROLE_2,
//fields
},
]
}
//Expected Output
{
//fields
"roleDiff":[ROLE_3] // roleList subtracted by set of roles present in siteList
}
//Script that works with Studio 3t
db.getCollection("SAMPLE_COLLECTION").aggregate(
// Pipeline
[
// Stage 1
{
$project: {
"roleList":1,
"siteRoles":{$ifNull: [{$reduce:{
input:"$siteList",
initialValue:[],
in:{$setUnion:["$$value", {
"$split": ["$$this.role", " "]}]}
}
}, []]}
}
},
// Stage 2
{
$project: {
"roleDiff":{ $setDifference:["$roleList", "$siteRoles"]
}
}
},
],
// Options
{
}
// Created with Studio 3T, the IDE for MongoDB - https://studio3t.com/
);
ArrayOperators.Reduce reduce = ArrayOperators.Reduce.arrayOf("siteList").withInitialValue(Collections.EMPTY_SET)
.reduce(SetOperators.SetUnion.arrayAsSet(StringOperators.valueOf("$$this.role").split(" ")).union(ArrayOperators.Reduce.Variable.VALUE.getTarget()));
ProjectionOperation projectionOperationOne = Aggregation.project().andInclude(/*Some fields,*/ "roleList").and(ConditionalOperators.ifNull(reduce).then(Collections.EMPTY_LIST)).as("siteRoles");
ProjectionOperation projectionOperationTwo = Aggregation.project().andInclude(/*Some fields*/).and(SetOperators.SetDifference.arrayAsSet("roleList").differenceTo("siteRoles")).as("roleDiff");
Aggregation aggregation = Aggregation.newAggregation(projectionOperationOne, projectionOperationTwo);
AggregationResults<SiteDiff> siteDiff = mongoTemplate.aggregate(aggregation, SampleCollection.class, SiteDiff.class);
The Java code above thows exception
org.springframework.data.mapping.context.InvalidPersistentPropertyPath: No property '$$value' found on class Did you mean: ?
The query works fine with Studio3T. My intention here is to get the difference between 2 String arrays "roleList" and "siteRoles" along with some other fields in the record.
"siteRoles" has to be derived from "siteList" which is an array of object. Since $addToSet works only with $group operation, I'm finding it difficult to extract the role from "siteList". I used reduce here combining it with setUnion. There problem was role is a string and I had to convert it to array. Only way I could find was to use $split and use " " as delimiter as I'm sure role will not have space.
Finally, the script worked in studio3t but the java version is not getting executed.

Answer 1

If you refer to a field in an embedded array, it should give you the entire array of those values.
The aggregation should only need 1 stage:
{$project: {roleDiff: { $setDifference: ["$roleList","$siteList.role"]}}}