mongodb aggregate a hierarchy to string in spring boot

Is there any simple solution to build a directiry path string from a hierarchy with aggregation in SpringBoot and MongoDB?

Directory.java looks like

@Getter
@Setter
@EqualsAndHashCode
@Document
public class Directory {
    @Id
    private String id;
    private String path;
    private Date createdAt;
    @DBRef
    private Directory parentDirectory;
}

(I use lombok)

The code that I tried is this:

public String buildDirectoryHierarchy(Directory directory) {
        GraphLookupOperation graphLookup = Aggregation.graphLookup("directory")
                .startWith("$_id")
                .connectFrom("parentDirectory")
                .connectTo("_id")
                .as("parents");

        ProjectionOperation projectionOperation = Aggregation.project()
                .andExpression("'$parents'").as("parents");

        Aggregation aggregation = Aggregation.newAggregation(
                Aggregation.match(Criteria.where("_id").is(directory.getId())),
                graphLookup,
                projectionOperation,
                Aggregation.project()
                        .andExpression(
                                "{$reduce: { input: '$parents', initialValue: '', in: {$concat: ['$$value', '/', '$$this.path'] } } }")
                        .as("fullPath"));
        AggregationResults<Map> results = mongoTemplate.aggregate(aggregation, "directory", Map.class);

        if (results.getMappedResults().isEmpty()) {
            return "/";
        }
        return (String) results.getMappedResults().get(0).get("fullPath");
    }

But I get the following exception:

Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception 
[Request processing failed: org.springframework.expression.spel.SpelParseException: 

Expression [{$reduce: { input: '$parents', initialValue: '', in: {$concat: ['$$value', '/', '$$this.path'] } } }] @73: EL1043E: Unexpected token. Expected 'rsquare(])' but was 'comma(,)'] with root cause

I'm not perfectly understand the code. Thanks for suggestions!

Solution

I created a working code:

    public String buildDirectoryHierarchy(Directory directory) {
        MatchOperation matchStage = Aggregation.match(Criteria.where("_id").is(directory.getId()));

        GraphLookupOperation graphLookupStage = Aggregation.graphLookup("directory")
                .startWith("$parentDirectory.$id")
                .connectFrom("parentDirectory.$id")
                .connectTo("_id")
                .depthField("level")
                .as("parents");

        Sort sortByLevelDesc = Sort.by(Sort.Direction.DESC, "level");
        SortArray sortArrayParents = SortArray.sortArray("$parents").by(sortByLevelDesc);
        AddFieldsOperation addFieldsStage = Aggregation.addFields().addField("parents").withValue(sortArrayParents).build();

        Cond reduceSeparatorCond = Cond.when(Eq.valueOf("$$value").equalToValue(""))
                                                            .then("")
                                                            .otherwise("/");
        Concat reduceConcatExpr = Concat.valueOf("$$value")
                                                        .concatValueOf(reduceSeparatorCond)
                                                        .concat("$$this");
        Reduce parentPathReduceExpr = Reduce.arrayOf("$parents.path")
                                                                .withInitialValue("")
                                                                .reduce(reduceConcatExpr);

        ExpressionVariable parentPathVar = ExpressionVariable.newVariable("parentPathCalc").forExpression(parentPathReduceExpr);

        ConditionalOperators.Cond finalSeparatorCond = Cond.when(Eq.valueOf("$$parentPathCalc").equalToValue(""))
                                                           .then("") // No separator if parent path is empty
                                                           .otherwise("/");

        StringOperators.Concat finalConcatExpr = Concat.valueOf("$$parentPathCalc")
                                                        .concatValueOf(finalSeparatorCond)
                                                        .concatValueOf("$path");

        Let letExpr = Let.define(parentPathVar).andApply(finalConcatExpr);

        ProjectionOperation projectStage = Aggregation.project()
                .andExclude("_id")
                .and(letExpr).as("fullPath");


        Aggregation aggregation = Aggregation.newAggregation(
                matchStage,
                graphLookupStage,
                addFieldsStage,
                projectStage 
        );

        AggregationResults<FullPathResult> results = mongoTemplate.aggregate(
                aggregation, "directory", FullPathResult.class
        );

        FullPathResult uniqueResult = results.getUniqueMappedResult();

        return Optional.ofNullable(uniqueResult)
                .map(FullPathResult::getFullPath)
                .orElse(null);
    }