[{"_id":"someId","videos":[{"_id":"1","name":"A","views":[{"userId":10,"group":"X"}]},{"_id":"2","name":"B","views":[{"userId":20,"group":"Y"}]}],"Assessment":[{"_id":"22","title":"Hello world"}]}]

db.collection.aggregate([{$addFields:{videos:{$map:{input:"$videos",in:{$mergeObjects:["$$this",{isWatched:{$anyElementTrue:{$map:{input:"$$this.views",in:{$eq:["$$this.userId",10]}}}}}]}}}}}])