db={"approval":[{"_id":{"$oid":"65214fd04aabc4486358cf6b"},"initiator":{"$oid":"65014c6303b5bf048f627b72"},"initiatorComment":"Please approve, as changes already made","history":[{"user":"65214fd04aabc4486358cf5a","action":"Request for Approval by rajesh dhoundiyal","tiemsamp":"2023-10-07T18:02:16+05:30","workbookId":"6511674ed0a5e9db5823051b","workbookTitle":"quaterly-3","gameTitle":"c1","message":"Request for Approval by rajesh dhoundiyal"},{"user":"65214fd04aabc4486358cf5a","action":"Approver action as READY_FOR_PTG taken by Manish Raj","tiemsamp":"2023-10-07T18:02:28+05:30","workbookId":"6511674ed0a5e9db5823051b","workbookTitle":"quaterly-3","gameTitle":"c1","message":"approve"}],"workbookId":{"$oid":"6511674ed0a5e9db5823051b"}}],"users":[{"_id":{"$oid":"65214fd04aabc4486358cf5a"},"sku":"almonds","description":"product 1","instock":120},{"_id":{"$oid":"65214fd04aabc4486358cf5b"},"sku":"bread","description":"product 2","instock":80}]}

db.approval.aggregate({$match:{}},{$unwind:"$history"},{$addFields:{"history.user":{$toObjectId:"$history.user"}}},{"$lookup":{"from":"users","localField":"history.user","foreignField":"_id","as":"user"}})