db={"channel":[{"_id":ObjectId("61efcbdc1aa27f83da47c93f"),"tags":[],"slug_history":["iny1Xik"],"title":"Pirate Chat","settingId":ObjectId("61408586b719c8ce89f08674"),"status":"published","lockedPageContent":"","slug":"iny1Xik","createdAt":ISODate("2022-01-25T10:07:24.144Z"),"updatedAt":ISODate("2022-01-25T10:07:24.144Z"),"__v":0}],"channelThread":[{"_id":ObjectId("61efcd5df82318884746eb80"),"threadImage":[],"parentId":null,"channelId":ObjectId("61efcbdc1aa27f83da47c93f"),"authorId":ObjectId("6177de8f8a5fd72a4f37b7db"),"threadText":"New Message","reactions":[{"authors":[ObjectId("6177de8f8a5fd72a4f37b7de")],"_id":ObjectId("61ef856432753c196382c37d"),"icon":"&#128528"}],"createdAt":ISODate("2022-01-25T10:13:49.033Z"),"updatedAt":ISODate("2022-01-25T10:13:49.033Z"),"__v":0}],"user":[{"_id":ObjectId("6177de8f8a5fd72a4f37b7db"),"image":"","tags":[],"pushTokens":[],"lastLogin":ISODate("2022-01-25T10:08:19.055Z"),"firstName":"dinesh","lastName":"patel","email":"dineshpatel@example.com","infusionSoftId":"784589","role":"user","__v":0,"settings":{"commentNotification":false,"commentReplyNotification":true}},{"_id":ObjectId("6177de8f8a5fd72a4f37b7de"),"image":"","tags":[],"pushTokens":[],"lastLogin":ISODate("2022-01-25T10:08:19.055Z"),"firstName":"kayle","lastName":"hell","email":"kylehell@example.com","infusionSoftId":"8475151","role":"user","__v":0,"settings":{"commentNotification":false,"commentReplyNotification":true}}]}

db.channel.aggregate([{$lookup:{from:"channelThread",let:{channelId:"$_id"},pipeline:[{$match:{$expr:{$eq:["$$channelId","$channelId"]}}},{$lookup:{from:"user",localField:"authorId",foreignField:"_id",as:"authorId"}},{$lookup:{from:"user",localField:"reactions.authors",foreignField:"_id",as:"users"}},{$addFields:{authorId:{$arrayElemAt:["$authorId",0]},reactions:{$map:{input:"$reactions",as:"r",in:{$mergeObjects:["$$r",{authors:{$filter:{input:"$users",cond:{$in:["$$this._id","$$r.authors"]}}}}]}}},users:"$$REMOVE"}}],as:"threads"}}])