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mongodbmongodb-.net-driver

Having Difficulty Using MongoDb C# Driver's Sample()

I am trying to get some random items from the database using the Sample() method. I updated to the latest version of the driver and copied the using statements from the linked example. However, something isn't working, and I am hoping it's some simple mistake on my part. All the relevant information is in the image below:

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Edit:

Greg, I read the aggregation docs here and the raw db method doc here, but I still don't get it. Last two lines are my attempts, I have never used aggregation before:

            var mongoCollection = GetMongoCollection<BsonDocument>(collectionName);
            long[] locationIds = new long[] { 1, 2, 3, 4, 5 };
            var locationsArray = new BsonArray();
            foreach (long l in locationIds) { locationsArray.Add(l); };
            FilterDefinition<BsonDocument> sampleFilter = Builders<BsonDocument>.Filter.In("LocationId", locationsArray);
            var findSomething = mongoCollection.Find(sampleFilter); //this works, the two attempts below don't.
            var aggregateSomething = mongoCollection.Aggregate(sampleFilter).Sample(25);
            var aggregateSomething2 = mongoCollection.Aggregate().Match(sampleFilter).Sample(25);
Solution

  • Sample is only available from an aggregation. You need to start with Aggregate, not Find. I believe it's also available in Linq.

    UPDATE: Looks like we don't have a method for it specifically. However, you can use the AppendStage method.

    mongoCollection.Aggregate(sampleFilter)
               .AppendStage<BsonDocument>("{ $sample: { size: 3 } }");