db={students:[{_id:"1",name:"stu 1",class:"7th"},{_id:"2",name:"stu 2",class:"7th"},{_id:"3",name:"stu 1",class:"7th"}],activities:[{_id:"1",studentIds:["1","3"],taskName:"Craft work 1"},{_id:"2",studentIds:["1"],taskName:"Craft work 2"}]}

db.students.aggregate([{"$lookup":{"from":"activities","localField":"_id","foreignField":"studentIds","as":"tasks"}},{$addFields:{tasks:"$tasks.taskName"}}])