Using MongoDB $arrayToObject and $map operators in java spring boot

Question

I have a query in mongodb with aggregation framework, which returns number of topics in my documents. This is done by utilizing the $group operator and the result looks like:

{
postsByTopics: [
        {
            "_id" : "Topic 1",
            "count" : 3.0
        },
        {
            "_id" : "Topic 2",
            "count" : 1.0
        }
    ]
}

I use projection to get a better shaped results

{
"$project": {
        "postsByTopics": {
            "$arrayToObject": {
                "$map": {
                    "input": "$postsByTopics",
                    "as": "el",
                    "in": {
                        "k": "$$el._id",
                        "v": "$$el.count",
                    }
                }
            }
        },

}

which returns much nicer shaped result:

{
postsByTopics: {
    "Topic1" : 3.0,
    "Topic2" : 1.0
    }
}

Now I would like to turn this projection in a java spring boot code. I found project().and(ArrayOperators.ArrayToObject.arrayToObject("postsByTopics"))

and

project().and(VariableOperators.Map.itemsOf("postsByTopics").as("el")

but I am struggling to combine them to achieve the desired result

Valijon · Accepted Answer · 2020-05-09 00:04:09Z

4

Spring-data syntax for "complex" query is not friendly.

Workaround 1: Implement AggregationExpression for .andApply method.

ProjectionOperation project = Aggregation.project().and(
    ArrayOperators.ArrayToObject.arrayValueOfToObject(
      Map
       .itemsOf("postsByTopics")
       .as("el")
       .andApply(agg -> new Document("k", "$$el._id").append("v", "$$el.count"))
    )   
).as("postsByTopics");

Workaround 2: Implement AggregationOperation with MongoDB JSON syntax

Aggregation agg = Aggregation.newAggregation(
    exp -> new Document("$project", 
               new Document("postsByTopics", 
                   new Document("$arrayToObject",
                       new Document("$map", 
                           new Document("input", "$postsByTopics")
                               .append("as", "el")
                               .append("in", 
                                   new Document("k", "$$el._id")
                                        .append("v", "$$el.count")
                           )
                       )
                   )
               )
           )        
);

answered May 9, 2020 at 0:04

Valijon

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3 Comments

karlitos Over a year ago

Many thanks, it works like a charm and your workaround example did help me solve some further issues with projection of map with posts-by-month, because the month is represented as number and the map operator allows nur strings as keys. For some reason new Document("k",ConvertOperators.ToString.toString("$$el._id.month")) gives me error (required string, found object) but new Document("k", new Document("$toString", "$$el._id.month")) works just fine

Valijon Over a year ago

@karlitos You are welcome. If you start to implement MongoDB native syntax, you cannot mix it with Spring-Data syntax. Try this: new Document("k",ConvertOperators.ToString.toString("$$el._id.month").toDocument(Aggregation.DEFAULT_CONTEXT))

karlitos Over a year ago

Many thanks, the .toDocument(Aggregation.DEFAULT_CONTEXT) was exactly what I was missing

kopeclu2 · Accepted Answer · 2022-01-25 12:09:04Z

Code above is NoSQL Query and down is almost similiar with MongoTemplate Cirteria Query. Maybe same propertieies are different, hope it will help someone ! Important is to save List as projection new property, i didnt find solution with arrayOf(List ... always inserted List in list with input: { elemetnAt: [[ .... whci hwas really wrong.

   $project {
     stats: {
         $map: {
             input: [ "2018-09-01", "2022-01-24", "2022-01-22", "2018-09-04", "2018-09-05" ],
             as: "date",
             in: {
             $let: {
                 vars: { dateIndex: { "$indexOfArray": [ "$days.day", "$$date" ] } },
                 in: {
                     $cond: {
                         if: { $ne: [ "$$dateIndex", -1 ] },
                         then: { $arrayElemAt: [ "$days", "$$dateIndex" ] },
                         else: { _id: "$$date", date: "$$date", distance: 0 }
                     }
                 }
                }
             }
         }
     }





project("_id", "days", "category").andArrayOf(timearray).as("daysEmpty"),
                project("category").and(
                        mapItemsOf("daysEmpty")
                                .as("date")
                                .andApply(new AggregationExpression() {
                                    @Override
                                    public Document toDocument(AggregationOperationContext context) {
                                       return new Document("$let",
                                                new Document("vars",
                                                        new Document("dateIndex",
                                                                new Document("$indexOfArray", asList("$days.day", "$$date"))))
                                                        .append("in",
                                                                new Document("$cond",
                                                                        new Document("if",
                                                                                new Document("$ne", asList("$$dateIndex", -1L)))
                                                                                .append("then",
                                                                                        new Document("$arrayElemAt", asList("$days", "$$dateIndex")))
                                                                                .append("else",
                                                                                        new Document("_id", "$$date")
                                                                                                .append("date", "$$date")
                                                                                                .append("distance", 0L)
                                                                                )
                                                                )
                                                        )
                                        );
                                    }
                                })).as("data")

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