In V3, this LINQ query:

Collection
   .AsQueryable()
   .GroupBy(doc => doc.Field)
   .Select(g => new { g.Key, Take = g.Take(6) })
   .ToList();

Generates:

[{ "$group" :

{ "_id" : "$Field" }

}, { "$project" : { "Key" : "$_id", "Take" :

{ "$slice" : ["$$ROOT", 6] }

, "_id" : 0 } }]

But fails with:

Command aggregate failed: First argument to $slice must be an array, but is of type: object.