C# MongoDb running an aggregate query directly from JSON

Question

I'm working on a service method that can run mongodb aggregate queries from a json input. The idea is that you would use Builders to generate a query, convert that query to json, pass it to the service to be deserialized and run. For find queries I was able to use Bson documents like so

    public string DoGenericFind(string queryDoc, string collectionName)
    {
        BsonDocument document = BsonSerializer.Deserialize<BsonDocument>(queryDoc);

        var results = _context.Database.GetCollection<dynamic>(collectionName).FindSync<BsonDocument>(document);

        if (results == null)
            return null;
        else 
            return results.ToList().ToJson();
    }

I'm having trouble finding a similar way to do this with aggregate. The only examples I'm finding around try to do something similar to this where they pass some kind of BsonDocument[]. However, the intellisense for my version of the driver(2.5) say that I need to pass a pipelineDefinition which I can't find good examples of how to use.

I opened an issue for this in Mongo's Jira.

HappyNomad
– HappyNomad

2021-02-06 00:53:20 +00:00
Commented Feb 6, 2021 at 0:53 — HappyNomad
– HappyNomad, Commented Feb 6, 2021 at 0:53

Filipe Donde · Accepted Answer · 2018-02-23 23:13:11Z

10

Look at this MongoDB Driver docs page:

Definitions and Builders

In the topic PIPELINE has a simple example that can help you. You will need create a simple code like that to use the pipeline definition.

PipelineDefinition pipeline = new BsonDocument[] 
{
    new BsonDocument { { "$match", new BsonDocument("x", 1) } },
    new BsonDocument { { "$sort", new BsonDocument("y", 1) } }
};

col.aggregate(pipeline);

Hope this helps!

answered Feb 23, 2018 at 23:13

Filipe Donde

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3 Comments

eatinasandwich Over a year ago

I looked at this exact same page and could've sworn I was passing the same thing. Must just be in a Friday end of the week haze.

Filipe Donde Over a year ago

It's happen with all devs!

Maxim Zabolotskikh Over a year ago

In the latest version of the driver it's now PipelineDefinition<BsonDocument /* or your collection type */, BsonDocument> pipeline = new BsonDocument[] { }

eatinasandwich · Accepted Answer · 2018-02-23 23:11:43Z

Found a good example right after posting this. https://groups.google.com/forum/#!topic/mongodb-user/Otg17LUE_7M

Which made my final solution look like

    public string DoGenericAggregate(string queryDoc, string collectionName)
    {
        var query = BsonSerializer.Deserialize<BsonDocument[]>(queryDoc).ToList();

        List<BsonDocument> list;
        using (var cursor = _context.Database.GetCollection<dynamic>(collectionName).Aggregate<BsonDocument>(query))
        {
            list = cursor.ToList();
        }

        if (list == null)
            return null;
        else
            return list.ToJson();
    }

Was pretty sure I had something crazy similar to this but was getting errors. But anyways, a List<BsonDocument> is implicitly convertible to PipelineDefinition<,> so that is passable to the aggregate function.

Luca Natali · Accepted Answer · 2022-07-02 18:45:14Z

1

thanks Filipe, while searching for the same issue I have find out that from MongoDB Driver 2.11 the PipelineDefinition syntax has changed requiring TInput and TOutput generics:

PipelineDefinition<BsonDocument, BsonDocument> pipeline = new 
BsonDocument[]
{
    new BsonDocument { { "$match", new BsonDocument("x", 1) } },
    new BsonDocument { { "$sort", new BsonDocument("y", 1) } }
};

The most recent docs is here

Thanks!

answered Jul 2, 2022 at 18:45

Luca Natali

3587 silver badges18 bronze badges

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