Spring boot MongoTemplate group without _id

Question

I want a single object from the collection that counts and calculates average.

The collection looks like this:

[{
    "_id" : ObjectId("617822b1d128026ff4c76a76"),
    "title" : "some title",
    "rating" : 4,
    ...
}
...
]

In mongodb, this is working for me:

db.getCollection('articles').aggregate([
{    
    $group: {
        _id: null,
        average: {$avg: "$rating"},
        count: {$sum: "$rating"}
    }
},
{
    $project: {
        "average": 1, "count": 1
    }
}
])

and returns

{
    "_id" : null,
    "average" : 3.60419139638543,
    "count" : 86.5005935132504
}

But when I try to do this in Spring Boot like this:

    Aggregation agg = Aggregation.newAggregation(
            Aggregation.match(
                Criteria.where("rating").exists(true)
            ), 
            Aggregation.group(null)
                    .avg("rating").as("value")
                    .count().as("count"),
            Aggregation.project("count", "value").andExclude("_id"),
            Aggregation.sort(Sort.Direction.DESC, "value")
        );

It says:

"java.lang.IllegalArgumentException: AggregationField name must not be null or empty!

What should I do?

Answer 1

I fixed this without passing anything to group

 Aggregation agg = Aggregation.newAggregation(
        Aggregation.match(
            Criteria.where("rating").exists(true)
        ), 
        Aggregation.group()
                .avg("rating").as("value")
                .count().as("count"),
        Aggregation.project("count", "value").andExclude("_id"),
        Aggregation.sort(Sort.Direction.DESC, "value")
    );