MongoDB lookup using 2 (or more) fields

I'm trying to lookup 2 mongo collection by 2 fields
I've tried creating an object / anonymous object as the local and foreign keys.

List<JoinedData> requests = await _persons
    .Aggregate()
    .Lookup<Person, Activity, JoinedData>(activities,
        x => new { Id = x.EntityId, Type = x.EntityType },
        x => new { Id = x.PersonId, Type = "Person" },
        @as: x => x.activities
    )

but I get an error:

MongoDB.Driver.Linq.ExpressionNotSupportedException : Expression not supported: new <>f__AnonymousType0`2(Id = x.EntityId, Type = x.EntityType).

is there a way to lookup like this?

thanks!

Writing it as answer since it's quite big. Some of the below can require more investigations:

1. Based on the documentation, I'm not sure whether local and foreign keys can be multi fields even in theory in query you use. Try looking at this lookup form with let argument.

2. I doubt it will help, but try using regular classes instead dynamic.

3. The error you see says only that the driver can't correctly handle provided expression, whether the query itself is valid or whether it can or can't be handled by server is a different question. Given the #1 I wrote above I doubt it will help, but you can try using a raw string-based configuration that will exclude parsing expressions at all and you can expect that exactly this query will be send to the server (again this query may be rejected by server, but you can try):

   List<JoinedData> requests = await _persons
     .Aggregate()
     .Lookup<Person, Activity, JoinedData>(activities,
         "#local option definition#",
         "#foreign option definition#",
         "#as option definition#"
      )
   

where values between # and # should be a lookup arguments in a raw MQL form, so think about it as placeholders for the MQL query that you can use for example in the mongo shell:

   db._persons.aggregate( [
   {
      $lookup:
         {
           from: "activities",
           localField: "#local option definition#",
           foreignField: "#foreign option definition#",
           as: "#as option definition#"
         }
  }] )

I would recommend opening mongo shell and build (and validate results) a query you're looking for there. Then just copy this query to the c# driver (in expression form if it works or with raw approach).

pay attention that raw string-based arguments won't be changed on the client side, so you should use field names as in your db structure (for example _id instead Id).