Lookup.kt

postCollection.aggregate<Post>(
                lookup(
                    "user",
                    listOf(Post::likes.variableDefinition()),
                    User::userId
                ),
                match(
                    expr(
                        MongoOperator.and from listOf(
                            MongoOperator.`in` from listOf(User::userId, Post::likes),
                            MongoOperator.eq from listOf(
                                User::userId, "_id"
                            )
                        )
                    )
                ),
                Post::likes.unwind(),
                group(
                    fields(
                        User::userId from Post::likes
                    )
                ),
                replaceRoot("_id".projection)

            ).toList()