db={"Candidate":[{"_id":"c1",appliedJobs:[{_id:"j1",date:ISODate("2022-09-15")},{_id:"j2",date:ISODate("2022-09-15")},{_id:"j3",date:ISODate("2022-09-15")},{_id:"j4",date:ISODate("2022-09-15")}]},{"_id":"c2",appliedJobs:[{_id:"j1",date:ISODate("2022-09-15")}]},{"_id":"c3",appliedJobs:[{_id:"j1",date:ISODate("2022-09-15")}]}],"JobPost":[{"_id":"j1","companyName":"Facebook"},{"_id":"j2","companyName":"Facebook"},{"_id":"j3","companyName":"Google"},{"_id":"j4","companyName":"Google"}]}

db.Candidate.aggregate([{$match:{appliedJobs:{$exists:true}}},{$unwind:"$appliedJobs"},{"$lookup":{"from":"JobPost","localField":"appliedJobs._id","foreignField":"_id","as":"JobPostLookup"}},{$unwind:"$JobPostLookup"},{"$group":{"_id":"$JobPostLookup.companyName","Applications":{"$sum":1}}}])