db={"Contacts":[{_id:"2341908342",fName:"Test",lName:"TestTwo",companyId:"61f4147b8415abfc04d09207",},{_id:"2341908343",fName:"John",lName:"Doe",companyId:"62847a16ee5c5047820415d2",}],"Companies":[{_id:ObjectId("61f4147b8415abfc04d09207"),companyName:"Facebook",domain:"www.facebook.com",},{_id:ObjectId("62847a16ee5c5047820415d2"),companyName:"Google",domain:"www.Google.com",}]}

db.Contacts.aggregate([{"$lookup":{"from":"Companies","let":{companyId:"$companyId"},"pipeline":[{$match:{$expr:{$eq:["$$companyId",{$toString:"$_id"}]}}}],"as":"companyLookup"}},{$unwind:"$companyLookup"},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$$ROOT",{companyId:"$companyLookup.companyId",companyName:"$companyLookup.companyName"}]}}},{$unset:"companyLookup"}])