Invalid Reference $$this for mongo array reduce arithmetic operation in java

Question

First of all here is project before reduce

How you can see there is a list called "1" with 3 fields (zone, duration, type)

I'm trying to reduce it and get the total sum of durations on 1_total_duration

But throws Invalid Reference for $$this.duration

Already tried to use ArrayOperators.Reduce.Variable.THIS.referringTo("duration").getName()

But since StringOperators.Concat works (converting first duration to string) I'm wondering why ArithmeticOperations throws "Invalid Reference '$$this.duration'!"

Don't want to use unwind since i will have multiple lists like "1" to do the same reduce process

Answer 1

Not a perfect solution but I managed to get what i needed like this

STEPS:

1- Convert "duration" to String

2- Reduce Array to a concatenated String separated with ","

AggregationExpression reduce = StringOperators.Concat
            .valueOf("$$value")
            .concat(",")
            .concat("$$this.duration");

    operations.add(project("1")
            .and(ArrayOperators.Reduce.arrayOf("1")
                    .withInitialValue("")
                    .reduce(reduce))
            .as("1_duration_str_array")
    );

3- Remove first "," (initial value is "" and is concatenated)

operations.add(project("1_duration_str_array")
            .and(StringOperators.Substr.valueOf("1_duration_str_array").substring(1))
            .as("1_duration_str_array")
    );

4- Split String with "," to get Array

operations.add(project("1_duration_str_array")
            .and(StringOperators.Split.valueOf("1_duration_str_array").split(","))
            .as("1_duration_array_str")
    );

5- Map elements to convert back to Double

operations.add(project("1_duration_array_str")
            .and(VariableOperators.Map
                    .itemsOf("$1_duration_array_str")
                    .as("d")
                    .andApply(ConvertOperators.Convert
                            .convertValueOf("$d").to("double")))
            .as("1_duration_array")
    );

6- Sum operation on Array

operations.add(project("1_duration_array")
            .and(AccumulatorOperators.Sum.sumOf("1_duration_array"))
            .as("1_total_duration")
    );