mongo-go-examples | mongo-go-driver examples

I don't know what I am missing but shouldn't I get the grouped results?

There are few things to point out here. The code example uses github.com/simagix/keyhole/mdb module for MongoPipeline. This function is supposed to take your string formatted MongoDB pipeline $group and converts into bson. This is a format that mongo-go-driver uses to send to MongoDB server.

Unfortunately the MongoPipeline function does not return any error that your code could catch, but the function is using encoding/json module to convert the pipeline into bson, and there are errors. The JSON parse errors that you're getting are:

• invalid character '$' looking for beginning of object key string => Because keys need to be wrapped with "
• invalid character '}' looking for beginning of object key string => Because there are extra comma(s) after }.

Due to these errors the MongoPipeline returns empty array [], which means undefined Aggregation Pipeline, and will return all documents in the collection.

The correct pipeline JSON string is (you can use jsonlint.com to validate)