Description

https://docs.mongodb.com/v5.0/reference/command/aggregate/#use-variables-in-let
Let variables only can be used in the aggregation expressions
e.g., you can’t do

db.cakeSales.aggregate(
   [
      { $match: { flavor: "$$flvr" } }
   ],
   { let: { flvr: "cherry" } }
)

it has to be

db.cakeSales.aggregate(
   [
      { $match: {
         $expr: { $eq: [ "$flavor", "$$flvr" ] },
      } }
   ],
   { let: { flvr: "cherry" } }
)

In docs it says “To specify variables that can be accessed elsewhere in the command, use the let option.”
Variables also cannot be used in other places like ‘from’ field for $lookup etc.
I think we need to clarify where exactly those work.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)