I couldn't find a way to update a nested array matching a specific query when using the aggregation pipeline for update operations.

Example :

I have the following data :

"_id" : ObjectId("5fd3ba7eb0b91523b424e612"),
"data-block-array" : 
[
  {
    "block-id"   : ObjectId("5fd3ba7eb0b91523b424e614")
    "metadata"   : {...},
    "data-value" : "Current value"
  },
  {
    "block-id" : ObjectId("5fd3ba7eb0b91523b424e616")
    "metadata" : {...},
    "data-value" : ""
  }
]

I want to update the data-value of the data block with "block-id" : ObjectId("5fd3ba7eb0b91523b424e614") for the document with "_id" :  ObjectId("5fd3ba7eb0b91523b424e612").

With the "classical" update (without pipeline), I can do :

db.getCollection('myCollection').updateOne(
  { 
    "_id" : ObjectId("5fd3ba7eb0b91523b424e612"), 
    "data-block-array.block-id": ObjectId("5fd3ba7eb0b91523b424e614")
  },
  {
    $set : 
    {
      "data-block-array.$."data-value" : "New value"
    }
  }
)

But with the following aggregation pipeline, that fails : 

db.getCollection('myCollection').updateOne(
 { 
   "_id" : ObjectId("5fd3ba7eb0b91523b424e612"), 
   "data-block-array.block-id": ObjectId("5fd3ba7eb0b91523b424e614")
 },
 {
   $set : 
   {
     "data-block-array.$."data-value" : "New value"
   }
 }
)

I get the error :

"Invalid $set :: caused by :: FieldPath field names may not start with '$'."

We also cannot use arrayFilters in an aggregation pipeline.

I've seen in the issue SERVER-40397 that you will never implement arrayFilters for the update with aggregation pipeline, but could you add the support for positional $ operator ?

I could use the other update version, but when you have in implementation (in C++ in my case) where we use polymorphism with a class method that returns a pipeline - which also supports things that we can't do with the other version - , it may be difficult to do so.