db={users:[{_id:"abc",name:"abc",group:1},{_id:"xyz",name:"xyyy",group:1},{_id:"123",name:"yyy",group:1},{_id:"rrr",name:"tttt",group:1},{_id:"eee",name:"uuu",group:1}],partnership:[{_id:"abc_123",fromUser:"abc",toUser:"123"},{_id:"rrr_abc",fromUser:"rrr",toUser:"abc"},{_id:"xyz_rrr",fromUser:"xyz",toUser:"rrr"}]}

db.partnership.aggregate([{"$match":{"fromUser":"abc"}},{"$group":{"_id":null,"exclude":{"$push":"$toUser"}}},{"$lookup":{"from":"users","let":{"exclude":{"$concatArrays":[["abc"],"$exclude"]}},"pipeline":[{"$match":{"$expr":{"$not":{"$in":["$_id","$$exclude"]}}}}],"as":"output"}},{"$project":{"_id":0,"output":1}},{"$unwind":"$output"},{"$replaceWith":"$output"}])