db={users:[{"_id":ObjectId("5f1efeece50f2b25d4be2de2"),"name":{"familyName":"Doe","givenName":"John"},"email":"johndoe@example.com","threads":[ObjectId("5f1f00f31abb0e3f107fbf93"),ObjectId("5f1f0725850eca800c70ef9e")]}],threads:[{"_id":ObjectId("5f1f0725850eca800c70ef9e"),"thread_participants":[ObjectId("5f1efeece50f2b25d4be2de2"),ObjectId("5f1eff1ae50f2b25d4be2de4")],"date_created":ISODate("2020-07-27T16:25:19.702Z")}]}

db.threads.aggregate([{$lookup:{from:"users",let:{participants:"$thread_participants"},pipeline:[{$match:{$expr:{$in:["$_id","$$participants"]}}},{$project:{_id:1,email:1,name:1}}],as:"thread_participants"}}])