Retrieve distinct values from Mongodb using Mongodb.Driver

Question

I am trying to return a list of distinct file types from a MongoDB collection using VB.Net (similar to C#.Net). Since each document has an unique id regardless of similar file types, the code returns duplicates. What am I missing here? I tried GroupBy but it didn't work.

Public Function GetAllFileTypes() As List(Of UXFiles)
        Dim m_List As New List(Of UXFiles)

        Dim db As IMongoDatabase = DatabaseService.GetDBcontext()
        Dim files = db.GetCollection(Of BsonDocument)("Files").Find(New BsonDocument).ToList
        m_List = files.Select(Function(_file) ConvertFile(_file)).Distinct().ToList()

        Return m_List
    End Function


    Private Function ConvertFile(fileDocument As BsonDocument) As UXFiles

        Dim file As New UXFiles With {
            .ID = If(fileDocument.Contains("id"), fileDocument.GetElement("id").Value.ToString, ""),
            .Name = If(fileDocument.Contains("name"), fileDocument.GetElement("name").Value.ToString, ""),
            .Type = If(fileDocument.Contains("type"), fileDocument.GetElement("type").Value.ToString, "")
        }
        Return file
    End Function

Answer 1

Distinct eliminates duplicate results from the result set. If you include _id of each document in the results, there are no duplicates since each _id is unique. You need to either project some fields (e.g. file type if you want to know what file types exist) or use something like $group with $first.