db={"users":[{_id:"AAA",fullName:"John Papa"},{_id:"BBB",fullName:"Robin Son"}],"company":[{_id:"1sd1s",Name:"Lumbar Company",User:"AAA"},{_id:"23s1dfs3",Name:"Patricia",User:"AAA"}]}

db.users.aggregate([{$lookup:{from:"company",localField:"_id",foreignField:"User",as:"companies"}},{$match:{$expr:{$eq:[{"$size":"$companies"},0]}}}])