db={"charts":[{"_id":ObjectId("5fd1ea22d605fede58184150"),"raid":ObjectId("5fd1ea22d605fede5818414e"),"description":"","password":"","created":ISODate("2020-12-10T09:28:02.661Z"),"messages":[]}],"raids":[{"_id":ObjectId("5fd1ea22d605fede5818414e"),"expires":"2020-12-10T09:34:10.808Z","invites_left":NumberInt("3"),"trainers":[{"_id":ObjectId("5fc375f7bf36c0adbd587578"),"username":"dsffdddsfs","host":true},{"_id":ObjectId("5fd257e3faafc618f3b29b07"),"username":"prova2"}]}],"trainers":[{"_id":ObjectId("5fd1e467d41457dc20a1f2f3"),"email":"test@test.com","password":"1","username":"dsffdddsfs","country":"ES","city":"Barcelona"},{"_id":ObjectId("5fd1e9acd605fede58184149"),"email":"test2@test.com","password":"1","username":"prova2","country":"ES","city":"Barcelona"}]}

db.charts.aggregate([{$lookup:{from:"raids",localField:"raid",foreignField:"_id",as:"raid"}},{$unwind:"$raid"},{$lookup:{from:"trainers",localField:"raid.trainers.username",foreignField:"username",as:"trainers"}},{$addFields:{raid:"$$REMOVE",trainers:{$map:{input:"$trainers",as:"t",in:{$mergeObjects:["$$t",{host:{$reduce:{input:"$raid.trainers",initialValue:null,in:{$cond:[{$eq:["$$this.username","$$t.username"]},"$$this.host","$$value"]}}}}]}}}}}])