db={dbchallenges:[{_id:ObjectId("66ea89ef454ef534047b1ba8"),tags:[{tagID:ObjectId("66e8a25ee39bd5ecdc6628a8"),position:0,_id:ObjectId("66ea89ef454ef534047b1ba9")}]}],dbtags:[{_id:ObjectId("66e8a25ee39bd5ecdc6628a8"),ro:"Tutorial",en:"Tutorial",master:true,__v:0}]}

db.dbchallenges.aggregate([{$match:{_id:ObjectId("66ea89ef454ef534047b1ba8")}},{$addFields:{tagIDsAsObject:{$map:{input:"$tags",as:"tag",in:{$toObjectId:"$$tag.tagID"}}},tagIDsAsString:{$map:{input:"$tags",as:"tag",in:{$toString:"$$tag.tagID"}}}}},{$lookup:{from:"dbtags",let:{tagIdsStr:"$tagIDsAsString"},pipeline:[{$addFields:{_idAsString:{$toString:"$_id"}}},{$match:{$expr:{$in:["$_idAsString","$$tagIdsStr"]}}},{$project:{_id:1,_idAsString:1,ro:1,en:1}}],as:"tagString"}},{$lookup:{from:"dbtags",localField:"tagIDsAsObject",foreignField:"_id",as:"tagObject"}},{$project:{tags:1,tagIDsAsObject:1,tagIDsAsString:1,tagString:1,tagObject:1}}])