MongoDB - Count number of object in array

Question

I'm trying to perform a single request to find the length of an array in a MongoDB document. In MongoDB, it can be done with an aggregation but to no success on my own.

db.mycollection.aggregate([{$project: { count: { $size:"$foo" }}}])

I've simplified the request (a lot) and think it can be optimized using the aggregate.

public int getArticlesCount(int id)
{
    int test = collection.Find(x=>x.Id==id).First().articles.Count;
    return test;
}

Is there any good way in the MongoDB C# to do this request?

Yong Shun · Accepted Answer · 2025-02-12 12:54:45Z

2

Your current approach is not wrong as the query is working with Fluent API/LINQ.

If you are looking for the solution with Aggregate Fluent:

return collection.Aggregate()
    .Match(x => x.Id == id)
    .Project(x => new
    {
        Count = x.articles.Count()
    })
    .FirstOrDefault()?.Count ?? 0;

Or if you are facing difficulties in converting the query into Aggregate Fluent, you can provide the query directly:

PipelineDefinition<Class, BsonDocument> pipline = new BsonDocument[]
{
    new BsonDocument("$match",
        new BsonDocument("_id", id)),
    new BsonDocument("$project",
        new BsonDocument("count",
            new BsonDocument("$size", "$articles")))
};

return collection.Aggregate(pipeline)
    .FirstOrDefault()
    ?["count"].AsInt32 ?? 0;

edited Feb 12 at 12:54

answered Feb 12 at 12:35

Yong Shun

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3 Comments

Riddsaw Feb 12 at 16:05

Hi, thank you for your answer, is there a good practive in the request to the database. Should I try to isolate the answer of the databasse to be the smallest necessary or to simplify the request as much as possible?

Yong Shun Feb 12 at 23:46

Hmm, sounds like an opinion-based. From what I understand from your comment, do you mean to simplify the query executed in the DB? From my perspective, the simplification in terms of reducing and only returning necessary data; avoids the complex computation to improve the query performance.

Riddsaw Feb 13 at 14:17

My thinking is that I dont need the whole object from the base to have the count. From my understanding, the 'collection.Find(x=>x.Id==id).First()' return the whole object and then we filter the articles and the count. But if the array length is counted directly in the database it should be optimal. Maybe it's opinion-based....

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