I'm trying to do a GroupJoin between two aggregation queries.

i.e. Collection1.OfType<TypeA>().GroupJoin(Collection2.OfType<TypeB>(), ...)

Trying to do this fails currently in ExtensionMethods.GetCollectionInfo as the inner expression is not considered constant.

$lookup looks to support this via the 'pipeline' parameter however there doesn't look to be any way to supply this through LINQ Join/GroupJoin translations.

I'm not sure if it's possible to add this support, I guess it would need to separate out the collection (constant expression) from the pipeline and forward the pipeline as the lookup parameter?

Thanks